If this is more appropriate in support, please move it. I am unsure if this is a technical (rather than behavioral) issue, per se.
Once updating to 1.14, my world has started repopulating many chunks. The chunks doing this seem to be ones that have either not been visited often, not been visited recently, and/or not been visited for a minimum certain amount of time. I'm not sure which, but the pattern observed is "chunks not gone to often are resetting, but ones in regularly visited locations are staying safe".
Some additional information in case any of it is pertinent.
The world is an older world, created in version 1.2.5.
I used a method/program called "Minecraft Land Generator" to generate a portion of the world roughly 20,000 x 20,000 blocks around spawn. The reason this was done was because when 1.7 released, it changed terrain generation. This method was used to allow updating without dealing with unsightly borders all over (and a nether tunnel was created to extend to a region beyond these borders, for new things). For those unfamiliar, the way this works is the program basically starts and stops a Minecraft server repeatedly, moving the spawn location between. When a server is started, all chunks within a radius are generated. This allows you to pre-generate chunks of a desired radius.
The world does not do this in 1.10.2. It starts once updating to 1.14.
Perhaps somewhat related, but water temples that were introduced in 1.8 sometimes randomly show up (no matter what biome, often in the middle of land...) when going on adventures to some of these pre-generated chunks that may not have been often, or ever, visited before.
What I THINK is happening, is the chunks I have NOT spent a minimum amount of time in, are, for whatever reason, being flagged as ungenerated, and the game is re-generating them. The game does the same thing to determine whether it is safe to generate water temples. This is a problem! With the water temples, it was an annoyance, but a rare one with a good chance of working around it (after the water temples was introduced into a new, unwelcome place, visit it for some minutes, and later overwrite said region files with the original ones, and this has limited applications as it is). With this, though, unlike water temples being a rare event, it is happening in ALL chunks not visited enough. Because of the terrain generation differences of pre 1.7 and post 1.7 versions, this is causing a swiss cheese-like network or terrain transitions all over the core part of the world.
How can I stop this? Can I somehow flag my existing chunks as "leave these alone, they are already generated, do not touch" to the game? If someone has a workaround that doubles as a fix for the water temple issue, too, that'd be fantastic, but the core issue is the important one. This will prevent me entirely from updating to 1.14 for the foreseeable future, which is a shame, as I lost a weekend of progress due to this.
If I had to guess, both issues have to do with the chunk "InhabitedTime" flag being less than a certain value; certainly, this explains ocean monuments, which can generate in existing chunks as long as the inhabited time is less than a few minutes, wherever there would be a deep ocean biome in 1.7+ (presumably, the generator uses the game's internal biome generator, as other structures do, since the chunks may not exist yet when a structure is being placed, which is normally done piece-by-piece as each individual chunk is populated, so it can't read chunks to check the biomes around the entire structure). Note that inhabited time was only added in 1.6; any chunks generated before then will start out at 0 when first loaded in 1.6 or later (this means that loading a pre-1.6 world in 1.8+ will have ocean monuments generating in chunks regardless of how long they were loaded).
Also, did you try "optimizing" the world in 1.13 first? This will convert all chunks to the new save format used by 1.13+; another optimization should then be done in 1.14.4 (the converter should be able to handle chunks generated as early as 1.2 since that was the last time the save format changed enough to require converting to a new format, which was mandatory and complete, unlike 1.13, which can convert chunks on-the-fly; the "optimize" feature was added to avoid performance issues caused by this. Any world created/upgraded since then could have such old chunks since versions in between only converted/updated any data as chunks were loaded while playing, hence they should have accounted for this (it is often advised to load a world in each intermediate version when upgrading across multiple versions but this only loads chunks around the player and world spawn - you'd have to completely re-explore the world to ensure that every chunk was loaded, which obviously becomes impractical for any sizable world).
Otherwise, I looked for reports of this on the bug tracker but I couldn't find anything; you could try making a report, including part of the world that is affected (one or two of the affected regions should be enough), which should help confirm if this is a bug or not (in the latter case Mojang presumably never considered the possibility of players using tools to pregenerate worlds, or assumed that few players would still have worlds old enough to not have inhabited time data).
Thank you for answering, and no, I updated to 1.14.4. from 1.10.2.
Loading all of the chunks of the central region of my world would be wildly impractical. The water temples are indeed showing up in spots where there would be one generated in that spot of the seed under 1.7+ generation, and only if the chunk hadn't been visited (or loaded?) before.
I am not sure the chunk repopulating is even limiting itself to that though, because I had to update UnMined when updating to 1.14, and that is when I noticed this...
I'm not sure what the Red areas are that never load mean, but I'm taking it as though UnMined is detecting something is amiss and can't load them. It seems as though those are the ones that have the potential to have the chunks repopulate. When first visiting the ocean in the first map, the repopulating started (that is when I stopped playing), and then the area behind it now shows as "no blocks".
For reference, here's the same world at those same spots in 1.10.2.