This might not be an LRT problem but I'd be very grateful for any help to pin it down.

I'm a new user. After several successful videos produced I had one looking very "jerky". It turned out the frames were badly ordered. I'm using a GoPro HERO8. When I switched from sort by capture time to filename, that fixed it. But how come? This is what I don't understand. I'll attach a screenshot of a small section of the list BEFORE I switched the sort order. The image is small and unclear but the middle filename is ...903.JPG.

By default LRTimelapse sorts bei the Date/Time information found in the Exif Data of the sequence.
Your screenshot shows that this has happened correctly. What is strange, that obviously the filenames are in a different order.
Since you say that the filename ordering is the "correct one" that reflects reality, obviously the camera wrote wrong exif timestamps into the images.
One reason I could imagine is the camera getting some internet time updates at that moment via a connected smartphone which altered the cameras time. But I'm only guessing here...

If you change the sorting to Filename Sort, make sure to remember to do this also in Lightroom before exporting!

Thanks a lot. I'll take this up with GoPro.

Sorry, don't like to argue, but if I sort by file timestamp in File Explorer I get the same order for some of them at least as in LRT, suggesting it is using system timestamp rather than EXIF.

Timestamp in explorer ist the file date, not the exif date the the camera wrote into the Metadata.

Sent mobile...

OK, so I don't understand what's happened here but I'm going to leave it for now. Hoping, wherever the prob is hiding, it might not bite me again!