Poster and play/pause buttons will not disappear

I am running W10 1903 in tablet mode.
I realise this is a niche use, but this is my tablet.
Playing a film the streaming is fine (over internal wireless), however even with full screen I cannot get rid of this. Apart from anything else it covers subtitles.
Any ideas much appreciated.

Is this using the browser, like Edge, in full screen mode?

Yes (both partial screen and full). You see in W10 tablet mode the only way (at least I have found) of making tiles is using Edge. I have a bunch of links to favorite pages, one of which of course is jellyfin.
I do not have this problem on Android or normal screen modenor using other browsers.
Is this a known issue?

There was a known issue with the web client in certain cases within 10.6.0-10.6.2, but it usually only affected Android. It should have been fixed by 10.6.3.

What version of Edge is it?