Have MacA and MacB running TM and backing up to the same Time Capsule. TC has the internal drive and two attached USB drives - same problem occurs on all of them. Both Macs are latest version - 10.15.7, and TC is the terminal version 7.9.1
MacB will frequently show a "Failed to eject diskX" message in the log. Then cannot mount that sparsebundle on MacA. This will continue even after complete power down and restart of everything - MacA, MacB, the TC, and the external drives - so some setting must be persisted to one of the drives, but what and where? If MacB is offline for a couple of days (wife's computer and not heavily used), and MacA has gone through a couple of daily power cycles, then suddenly all of the bundles can be mounted. When MacB is turned on again, and generates some new backups, then the same problem recurs.
Have reported to Apple feedback with full discussion and logs https://feedbackassistant.apple.com/feedback/7739401 but so far crickets.
Will sometimes (rarely) have the same problem with a sparsebundle for a MacA backup also not being mountable on MacA - this usually clears after one reboot. Note that the remount issue occurs when trying to use Finder, or from the command line with hdiutil or open (using DiskImageMounter internally) - usually get a "Resource temporarily unavailable" message. Time Machine itself never seems to have any problem re-mounting these bundles for the next backup.
Suspect some XATTR setting, but various experiments have all failed.
Any suggestions? Thanks