Many people have reported this. You can try turning off the Loupe Info overlay using View > Loupe Info > Show Info Overlay, which works for some. You can also try setting Preferences > Performance > Use Graphics Processor to Off.
But in general, LR 6 is no longer supported by Adobe, and a number of problems with LR 6.14 and 5.7.1 have been reported on Mac OS 11:
Beware that you won't be able to reinstall LR 6 on Mac OS 10.14 or later because its installer is 32-bit and later versions of Mac OS don't run 32-bit programs. (Not that reinstalling will likely help these problems.)
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