.mov vs m4v and not showing videos in html - Big Sur

Hi ... I recently moved from High Sierra to Big Sur, and changed from iPhone 7 to iPhone 12. I have seen a couple minor differences BUT one that is causing me problems, I am hoping someone can help.

I take a video and import it into Photos. It comes in as a MOV (same as before) but when I export to the file system, it stays as a .mov file (it used to export as an m4v file). I then try to show the video via html using this code:

<video width="800px" controls="true">

<source src="IMG_1234.m4v"/>

</video>

It used to play fine (both video and audio).

With the same browser (both Firefox and Chrome), I get the audio but no video at all.

It plays in QuickTime on the Mac and also in Pictures, but not with my HTML.

Anyone can help??

Thanks in advance.