While debugging bug 209719 I observed that sometimes the video is rotated 90 degrees in web.whatsapp.com. Sometimes it happens for the poster image as well.
Maybe this was fixed by https://trac.webkit.org/changeset/275412/webkit ?
I have been trying to reproduce this issue on a recent trunk (@276497, from April 23th 2021) and haven't been able to reproduce it in Flatpak + MiniBrowser. Surprisingly, after reverting the patch pointed by Philippe (had to solve some conflicts to do it) I couldn't reproduce the issue either. Should we close this bug? Fernando, can you still reproduce it?