Hi,
Based on your description , the server are listed in wrong computer group but the "Group membership" are showing correct .
I also did a test in my lab .
1. I changed server behavior to "manually moving" .
2. move a computer to anther computer group (new10)
3. change "manually moving" to " GPO / reg targeting" (and , re-detect WSUS server) , it result into "group membership" and "current computer group" mismatch :

I'd suggest you perform "rename/delete software distribution folder" , "reset
wsus agent components" , "re-configure GPO setting" for a problematic client to check if that computer will be listed in correct computer group (after a successful report -- please check the date of "Last Status Report" ) .
EDIT:
After I restarted the WSUS server , "mismatch" disappeared .
Best Regards,
Elton
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