Similar to #1915 and referenced as fixed here: #1849

However... I'm experiencing this in 1.21 with just Carpet Mod installed (removed all other mods).

To replicate, create a new world, place a repeating command block / no redstone required, with this command:

execute unless entity playernamehere run player playernamehere spawn

then /kill playernamehere

that player will remain visibly in the world, not be in the player name list, but will still be detected by the command block you had placed

if you leave and reenter the world, you'll get the multiple messages about the joining player.