It's not just the player name that no longer works. You also can't use entity UUIDs or @a/@e/@r/@p/@s.
For example:
/tp @e[type=minecraft:goat,limit=1]
/tp 2cb92653-8309-4e5f-a625-fcef5ad3e52f/tp @e[type=minecraft:goat,limit=1]
/tp 2cb92653-8309-4e5f-a625-fcef5ad3e52fTested in 21w15a
I was able to reproduce the bug