I'm the operator of a private server running 1.14.2 in charge of whitelisting members. Recently I have been unable to add many players with apparently valid IDs (they are able to create Java edition launcher profiles with the ID and connect to other servers) to the whitelist. "/whitelist add <player-id>" runs for about 15 seconds (successful adds run for less than a second) and then responds with the message "That player does not exist". I don't know exactly when the problem started, but it was prior to the 1.14 upgrade.
The only clue I've found is that none of the player IDs that have recently failed can be found on https://namemc.com/ (which I found mentioned in an old forum post), while IDs that can be added to our whitelist can be found. I don't know how NameMC works, but is that significant?
UPDATE: One additional detail, we are running a Vanilla server.
I am also one of the users attempting to get white listed on this server. My account was only created back in march and has never been used for multiplayer at all, let alone any sort of ban. Also my client is fully updated although I doubt that's relevant. I can provide any info requested to verify my account.