After hours of troubleshooting and turning various mods on and off, I seemed to have found the culprit... and it's not one you'd expect right off the bat to deal with the rendering issues.
Not Enough IDs ( NotEnoughIDs) , which (while I don't know how it does it, does it) changes the amount of ID's for blocks to more than 4096, seemed to be affecting blocks being rendered. This also had a side effect of re-loading worlds and having rendering issues (without restarting clients or with).
Turns out that while I am running nearly 300 mods, I did not need that one, and once turned off, the issues have gone away.
Hopefully this helps anyone else who comes across it in the future