It could be Facebook. They might not have updated their app to support the new iPhones.
True. I jumped to a conclusion there.
What I know for sure is the issue is not solely caused by IOS13 (or, in particular, 13.1.2) since both test devices are using it.
The iPhone XS used has been successfully used many times to do exactly this - to upload panoramas to FB which are then recognised and visible as 360s.
The iPhone 11 Pro simply does not work, in a literal side-by-side test (my wife standing next to me taking the same photo on her XS that I took on my 11 Pro at the same time with the same IOS version); hers uploaded properly, mine didn't.
So yes
@scruffypig I mispoke, it's a compatibility problem between the 11 Pro and FB which, I presume, means there are changes to file structure / size / EXIF info for the 11 Pro which require changes by FB and they have yet to recognise the problem and make changes.
At least I'm not going mad so I'll just sit and wait for them to fix it.