Video filmed with iPhone not compatible with iMovie on Mac?

Is this true? I have filmed a clip on my iPhone 8, iOS 14.01. I have transfered the clip via Airdrop to my MacBook Pro, running Catalina 10.15.7. I then tried to import the clip into iMove 10.2.1, but I just get a black screen when previewing the clip or playing it from the timeline. The audio works, but the clip is of course useless without the images. I read somewhere that I should try to change the settings on my phone, so that it preserved the original format when transferring to a computer, so I did that, but still the same. I also tried with letting the phone change the format automatically, same problem. I also tried to download the clip from iCloud, to see if there was a difference between that version and the one on my phone, but doesn't seem that way, because the same problem occurred. It sounds insane that Apple would not make their own software on their own computers compatible with videos shot on their own phones... but to me it seems to be this way. Or am I missing something?