I was able to reproduce the bug
Seed: 9055926848729742660
Command: /place structure minecraft:desert_pyramid 164 -43 31
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-fcef5ad3e52fTested in 21w15a
Same here. Is it platform specific? eg. Mac/ PC?