In the video, I provided exactly how the error occurs.
Now I will try to briefly describe it.
If you have a server with a white list, and after starting the server you added a nickname to the white list, and immediately go to the server, then your nickname will be fixed in the white list.

If you were added to the white list, but you entered the server after restarting it, then your nickname disappears from the white list, and this is due to the fact that your XUID was missing from the white list

XUID appears exactly after you enter the server, but only on the condition that you entered BEFORE restarting. If you logged in after the restart, then you need to add it again.

The error has been tested on several devices, and it is the same everywhere.

https://drive.google.com/file/d/1NihE4t6RbCTwYgCoCywKWshGUYtljbLl/view?usp=sharing