Did some further digging. It seems only iPhone videos created between November 2018 and September 2019 have green thumbnails for me. Hope this helps out the troubleshooting.
I have the same issue. I think I have it isolated to movies I recorded on my iPhone from September 2019 and earlier. I don't recall what version of iPhone I was on then (maybe a 6S?), but in September 2019 I got an iPhone XS. Movies recorded after getting that phone don't have the green...