Well, it certainly sounds like something is wrong on the iMac. You could perhaps try resetting the preferences file on the iMac (press and hold Shift+Alt/Opt when launching the Lightroom app), then see if that changes anything (also make sure you set the preferences back the way you had them).
If resetting the prefs doesn't fix the issue, if that was happening on one of my systems I'd reluctantly conclude that the local library on that system needs to be reset. Basically, deleting the local library will cause it to be recreated automatically from the master cloud catalog, so all you lose is the time it takes for all the thumbnails/proxies and catalog file to re-download. But before doing that you need to consider what your preferences are set to, particularly on the Local Storage tab, i.e. do you have either of the local storage options (smart previews or originals) enabled? If the smart previews option is checked, the length of time to re-populate the local library will take quite a bit longer as there'd be 77k smart previews to download (somewhere between 75GB and 150GB). If you have the "store a local copy of all originals" also checked, I'd assume that these are not stored in the local library but are instead stored on a separate drive.....in which case they'll not need to be re-downloaded provided the location in the Preferences>Local Storage tab is still set to the external drive (resetting the prefs earlier would have changed that back to the default location, so make sure it's changed back!
However, before doing that deletion it might be worth hanging on to see if any other user has a better idea about what might be wrong and how to fix it. I know that
@Victoria Bampton is a heavy Lightroom user, so she may have seen this problem in the past.