I installed it with anki-2.1.53-linux-qt6.tar.zst on the GitHub releases. anki-2.1.51-linux-qt6.tar.zst works fine.
I’ve already seen this bug on this forum, recently, but I can’t find it anymore IIRC correctly, the problem was due to a Qt6 bug, and switching to Qt5 solved it.
I’ve added this to Browser sidebar appears as small box in top left · Issue #1902 · ankitects/anki · GitHub
As additional information, this bug this bug is more likely to occur when there are a large number (>5000) of cards. However, there may be other causes as well. I haven’t seen the issue recently as I’ve deleted a lot of unused cards.
As can be seen on the link above, @hkr has contributed a fix that will be in the next release.