I have the same issue on my MacBook Pro 16'' from 2019 with macOS 11.6.3.
Installing the previous Safari beta 15.3 fixed it temporarily, but the issue came back after a few minutes of arranging tabs in tab groups.
After installing Safari 15.4 beta 1 the issue seems to be fixed again. I'm not sure if it will last, though.
ToBrobi
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