This was for 1.19.4:
MC-261082 Player team entries are stored as player names - Jira (mojang.com)
It says resolved, but the error is still there in 1.20.1.

Steps to reproduce:
1) On a multiplayer server, create a team and do /team join <teamName> <playerName>
2) Go on the Minecraft website and change the player name.
3) Log back into the server and observe

Observed:
The player is no longer on the team. 

Expected:
The player is still on the team. 

This is most likely due to storing team data using player names instead of UUIDs.