UPDATE! In case someone comes across the same problem, it seems to be either just a media encoder issue, or a media encoder/BigSur issue.
My laptop had Catalina still on it, converted a mp4 and imported into animate no problems.... then airdropped that mp4 to my desktop with BigSur and the same mp4 worked fine. Both my laptop and desktop were running current versions of Animate and media encoder.
No solution though, unless you're lucky like me and happened to not update your laptop in a loooong time.