Yeah I have to chime in as well, just started working with my first multi-part fixture and this is absolutely driving me bonkers. Especially because it kills the ability to use multi-function Fx presets that combine parameters that are across the two fixture parts.
But honestly I think it should be fixed at a lower level because this same exact issue comes up if you take two different fixtures like a LED PAR along with a moving light and you should be able to control all parameters of both selected fixtures, the software should just recognize not all parameters might be sent to all fixtures.
Funny thing is if you do a [.][enter] to select ALL patched fixtures it works just fine.... just doesn't work using the fixture selector or fixture group presets. The fact that selecting all patched fixtures works and the other doesn't really seems like a bug.