Hi @JoeViking245 - I'm back again (like a renegade master...) and have just completed some extensive testing.
Tried lots of things - renaming the game without commas, running it locally (it currently runs from a NAS), 4:3 aspect ratio (currently I have it 16:9) etc....
Finally found that if I turn on "Skip startup animation" in Xemu (see image below) it works a treat! The filename and all other things are the same but this is now working great.
So, it's a strange fix and I'm not sure of the cause but just wanted to post back here in case it helps someone else in the future. And a big thanks to you Joe for pointing me in the right direction with the logs etc. Very much appreciated.
Johnny T's post in Strange behaviour on Xemu - some games work launched directly but not through LB? was marked as the answer