Before I had all instruments, I vaguely remember in GUI you can hide the demo instruments so they don't accidentally get selected (pressing "N" on computer keyboard) - a while ago - less certain how that behaviour translates to headless.
There may be rare limitations in the way some mini systems (packages etc) interface with Pianoteq (software issue like this exists in a recent project I see - the dev is aware etc.) - like not having compatibility above "Standard" version - maybe there's something of this type going on with your Raspi (like certain API call not in package).
Maybe someone with a different system has a different experience? Would be interesting.
In the case you want to select the demo instruments, even if just for pleasure, I genuinely recommend buying those instruments you like, even so more funding can reach the modelling team, thus more models are more likely to come, rather than fewer and quality continues etc.
I'm one of those who have Studio version so that I can have all instruments uninterrupted as a best value proposition while supporting the best software I've personally come across. I can say that I've found it's worth it. This software is one of the very few in this category for me (long time software user, from freeware through to paid).
I outright begrudge paying for nonsensically overpriced software/updates/upgrades and Pianoteq is just not in that category.
If purchasing the demo instruments is not an option though, I'd contact Pianoteq support, to see if there's a way to do what you ask (could be a feature to drive purchase, rather than a bug!), or maybe contemplate saving for these ones you like to use, regardless if for just pleasure - people pay more for roller-coaster tickets for pleasure too after all and I get so much more than 2 minutes fun out of every instrument and no woozy after effects
Pianoteq Studio Bundle (Pro plus all instruments) - Kawai MP11 digital piano - Yamaha HS8 monitors