I have Big Sur installed and the latest Mac App Store version of Enpass (though I also tried this on the slightly older 6.7.0 beta, too).
I use a different clipboard manager, Flycut (link to the Mac App Store version, 1.9.6.
I had no problems copying anything from Enpass into an unmodified clipboard (with no clipboard manager active at all) and I also had no problems copying anything into Flycut. I tried copying from an item I was editing in Enpass and from a standard login item and note (in display mode).
Flycut has Accessibility permissions, but only that.
I tried this with and without the "clear clipboard content" feature enabled. With it enabled, I didn't have any issues, but the content would disappear from the Mac clipboard (but not Flycut) after the specified amount of time.
I also have another machine with Catalina and Enpass and Flycut and I didn't run into problems.
You mentioned several other programs with clipboard managers, so I tried the ones I could without having to pay for them. I was able to try Paste.app and Keyboard Maestro 9.2.
My tests show that Paste.app doesn't import the clipboard contents from Enpass or 1Password, even after I removed the rule restrictions (but the copy DID appear in the Mac's native clipboard until the clear content time was reached). I was able to get Keyboard Maestro to work (though for some reason I noticed it took like 15-20 seconds for anything I copied to appear in the clipboard. I had to dig through the packaged binary to find the KM helper app and add it to the Accessibility section of the Security preference pane. For some reason, only the main app itself automatically appeared there by default and I wasn't able to copy anything until I added the helper app.
I couldn't try Alfred because the clipboard features seem to require the Power Pack and I wasn't going to pay for something I don't intend to use.
So to sumarize, for me:
I was able to copy stuff from the latest version of Enpass (and the beta) successfully both without any clipboard manager enabled. Copying and pasting content worked fine on Big Sur and Catalina with or without the "clear content" feature enabled.
With Paste.app I was able to copy and paste stuff using the standard keyboard shortcuts so long as I didn't interact with Paste.app in any way before pasting the content. If I interacted with Paste in any way or if the Mac clipboard was cleared, Paste was unable to paste anything.
I was able to get Keyboard Maestro working (with a delay) so long as I added the helper app (located inside the KM app package) to the Accessibility permissions.
Flycut works just fine with Enpass.
The results above didn't change if I ran them on Catalina instead of Big Sur.
@ButisitArt57 Can try you try shutting down all of your clipboard tools and just try a standard copy and paste again? Keyboard Maestro can be particularly tough to shut down completely, because the helper app doesn't stop if the main app is quit. If you need a clipboard manager, perhaps try Flycast and see if that works for you? I'm not sure if Enpass is marking the clipboard content in a way some clipboard managers don't like, but it sounds like you might want to get the clipboard manager developers involved. Perhaps they can help narrow down the problem you're having.