Same issue with a late 2014 iMac 27" running BigSur. The machine seems to run OK so far (bought it used), but when I run Disk First Aid seeing a large number of the: warning: MT mapping (0x40000002d013b -> 0x4fdcef, 1, C) is not completely referenced warning: MT mapping (0x400000037350d -> 0x11657e, 1, C) is not completely referenced warning: MT mapping (0x400000037350e -> 0x4083da, 3, C) is not completely referenced .... I could try reinstalling the system from scratch, but I might just want to swap out the Fusion drive for an SSD (which might be a pain), or just get an external SSD. Not sure that is the correct way, but there is a YouTube Video about how to replace an internal drive: https://www.youtube.com/watch?v=YZv6hR3VgyY