This is a “very low bug” i think.
Any capable user can avoid this issue manually opening the video, converting it with another software and use it with flavours.
But why my phone cant share the original mp4? Neither Album (sony app for gallery) nor whatsapp or Twitter show generated movies, and if i do the manual search, the file shows up grayed out (see screenshot).
Perhaps is Android 8.1 that lacks support for this codec?
Thanks for all!