We have been running the server almost 5 years, upgraded and optimized the world with each update,  and never had the problem before, except for a couple of days when authentication servers were apparently down in 1.18.1 and then there was not work around.  But this has been happening for the past 2 and 1/2 weeks so I doubt the Minecraft authentication server are down that long.

1. run command "/whitelist add playername"
2. response "that player does not exist"
3. Player is know to have valid Minecraft account which has been migrated to Microsoft account.
4. tried adding  uuid and name manually by editing whitelist.json file. The name is in the whitelist.json, but the player sill gets message "you are not whitelisted on this server."
5. Tried removing old whitelist, json and started to add players to new whitelist. json,got same response for all players already on the old whitelist.json file, Put back the original whitelist.json

Work around :

1. Make sure player is standing by (via phone msg , discord, a friend of the player relaying messages via discord, etc ), while Server OP is also logged into the server world as a player.
2. OP runs command "whitelist off"
3. Player joins server
4. .OP runs command "/whitelist add playername"
5. Response "Playername added to whitelist
6. OP runs command "/whitelist on"
7. Player can successfully join after that.

It is nearly impossible to coordinate and leaves the server vulnerable during the time the whitelist is down. I used to be able to add players to the whitelist without them even being online, much-less actually being on the server.  Also I contacted the Hosting Company Support and they were at first shocked to learn about the problem and said it should not be happening. However, they were able to repeat the same results I got, as well as the work around.