Hotbar shown in loading terrain screen

Here are two videos of it happening (one is in slow motion)

Can confirm for 1.21.6-pre2: Additionally this does not only apply to the hotbar, it applies to many more UI elements (Scoreboard, Bossbar, Chat, Titles).

This also applies to switching “subservers” that are located behind a proxy.

On a local server with much power the issue is barely visible, to reproduce it for a longer period of time I’ve spun up a testing server with limited CPU power using

cpulimit --limit 100 -- java -jar server.jar nogui