This stuff can be tricky. I had the same problem when trying to use the GM mode in my old Alesis QS8. It would play Biab tracks just fine but as soon as I tried to change to a specific sound so I could play along, the Alesis would jump out of GM. There might be an answer, don't know but just spend $40 for the Forte DXi software synth and be done with it. The Forte probably sounds at least as good if not better than the GM sounds on the Ensoniq. What happens is you use the Forte for all the Biab tracks then you're only using the Ensoniq for playing one sound on the thru channel.
Ensoniq was bought out by EMU some years ago, then EMU was bought by Creative. The board is actually pretty decent. It's a fully weighted piano action 76 key workstation and it's good for your kids to learn piano technique on. I almost bought one but got the QS8 instead because the piano sound was way better and they both use the Fatar keyboard action. While the Ensoniq is fully weighted, the piano sound was it's weakest point. A lot of the other sounds are pretty good though. I remember liking some of the organs and Rhodes on it.
Keep learning it, it's not some junk keyboard and Ensoniq was known for having the best on board sequencer in the business. As a PC tech I'm sure you have some A drives and 3.5 floppies laying around. You can burn your Biab songs as Standard Midi Files (SMF) and put the disc in the drive on the Ensoniq. I know you can play along with the midi's that way. This works both ways, btw. You can use the sequencer to create a midi file and load it into Biab or better yet Real Band for further manipulation. You have a lot of possibilities with that Ensoniq.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.