"Continue watching" between devices seems to "corrupt" content

[Spymaster 5]

This is a weird one!  I watched Emby via Tailscale on my Amazon firestick in a hotel room last night - perfectly fine playback - and exited two shows mid-way through.  However, when I came to continue watching those shows tonight on my LG C5 TV app neither of them worked properly.  One of them - a 4 hour horror documentary - stuttered, froze, and continually attempted to "recover from error" (according to stats for nerds) even when I attempted to restart it - and the other - an episode of Al Hail King Julien won't play at all.

The MP4 files - I have to assume - are fine but somehow Emby has "corrupted" playback between one device and the other.

I downloaded and re-uploaded the horror documentary to my NAS but the errors were repeated.  I think I may have fixed it now by copying the MP4 and uploding the copy.  It appears to work okay from the start.

I'm using the very latest version of the Emby app for Synology which I downloaded tonight, and the LG TV app is also up to date.

BOTH PROGRAMMES PLAYED FINE TONIGHT ON THE EMBY APP FOR ANDROID, HOWEVER.  THE PROBLEM APPEARS UNIQUE TO THE LG TV APP

Do you know what might be causing this problem?