DRM issue? Are you sure you are using the same userid that is on record for purchasing the movies? (though this shouldn't crash the app- I think). Are you perhaps logged in with one userid on AppleTV and a different one on iMac? AppleTV MIGHT be playing a version on iTunes server (streaming) instead of the version you've downloaded to your iMac. Thus, it could seem to play fine and be up to entirely corrupted on your hard drive.
"usually playing through AppleTV" implies that you perhaps have not tested one that won't play right now. Definately try to play one that is failing to play on your iMac but will play on your AppleTV (right now, not assume the file is fine because it played in the past). If it DOES play just fine on AppleTV now, are you sure you are playing the one you've downloaded and AppleTV is perhaps not streaming the 4K version from iTunes servers? Are you using the Computer app on AppleTV or something else? In this hypothesis, you could have a corrupt copy on your iMac that your AppleTV is basically ignoring. Thus, the movie plays fine on AppleTV but won't play on iMac because each is playing a DIFFERENT copy of the movie.
Have you tried deleting one that won't play on iMac and downloading it anew from iTunes and then try playing the new download? This is another version of the "corrupted" hypothesis but also addresses the first one (different userid DRM) too.
Are you sure you are trying to play the one downloaded? As things are set up now, the downloaded one is up to 1080p but if the movie has a 4K version, it might be trying to stream the 4K version instead. The idea here might be something in the internet connection, wifi bandwidth, etc. (you think you are trying to run the version you've downloaded but you have things set up to substitute the 4K version via streaming instead... and maybe don't have the bandwidth or something like that?) Is the iMac OLD and maybe can't quite handle the 4K version?
Are perhaps other people hogging up a bunch of your bandwidth and there's not enough left over to stream the movie?
Are your rips & downloads perhaps (probably) stored on an external drive due to the amount of space many of them demand? If so, are you sure the external is attached when you try to play it? Monterey seems to have a bug about "unexpectedly eject"ing external drives... even while being used. If that happens, the movie will still appear in the list but there's no file to play when you try to play it (as if you've disconnected the drive). If this scenario, are you giving perhaps a sleeping external drive time to fully wake up?
Does TV(app), Preferences, Files, Media Folder location point to the right place?
Does the list of problematic video files actually point to the correct files? Try deleting one that won't play in TV (app) and then drag and drop it back into TV (app) and try playing it again. For example, if you've maybe replaced your media drive with a new one, you likely have files stored in TV app looking for the old drive instead of the new one. Due to "unexpectedly eject" Monterey issues, I had to temporarily retire my old external drive and replace it with another. ALL of the media files were no longer pointing to the right files so none would play. Solution: I deleted ALL of them in TV app and then dragged and dropped them back in from the new drive. This corrected the file addresses and they all work now.
If you've maybe replaced the media drive some time ago but the ones that are failing you are still pointing to the previous drive, that would create a situation where some files play (those since added to the new drive) and some don't (because those still point to the old drive).
I just tried a purchased, DRM'd, downloaded movie in Monterey 12.4 and, after the spinning beach ball while the new external woke from sleep, it did start playing. So at least ONE example implies that things are not completely broken.
There's a LOT of possibilities. Try the above and share what happens. Be detailed so that "we" can better understand what is happening there. That will help narrow in on whatever it is.
And experiment to rule out as much as you can. For example, test by SYNCing (not streaming) a movie from your iMac that won't play to an iPhone or iPad and then try to play it on the iDevice in airplane mode (so you know you are not streaming a copy from iTunes servers). Will it play on the iDevice? This would throughly rule out a corrupt file scenario.