What is the current version of BigSur ? if not updated , update it to 11.6 and check
If updated, you can apply First aid to Mac HD via Disk Utility.
also you may try to reinstall Mac OS from Recovery mode and check. it will not affect Data just reinstall OS.
Anyhow safer side , keep backup of data onto external storage or Cloud storage.
Second option:
Yes you can downgrade to Catalina if you wish to.
Backup all data, go to recovery mode and erase Mac HD and then install Mac OS Catalina via External Boot .
Make sure that Allow external boot devices from Startup security if the mac has T2 security chip.
For further assistance you can contact Apple Support .
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