@ jcazmusic
You actually ask about how every application work without you seem to know anything about them. You somehow exspect them all to work the same way and do the same thing no matter what, but that's not the case.
Let me give a couple of examples. The CS-80s sustain work just like the original CS-80s sustain did. It's a kind of hold that can work different for each preset, and in different ways. It's not a simple note hold like you apprantly wan't. You have actually some parameters for it๘s settings.
And while Matrix 12 does have added a "normal" sustain like i think you talk about, then it actually does'nt allways work that well, except for some tasks, but you can still add sustain pedal the same way as the original. That too is a more detailed hold, and it work fully.
As said, then please ask about the individual applications in the applications own forum. But please try the applications out first.
Why do you think all is bad, just because Mellotron does'nt have hold? Have you tried other full Arturia applications, where it does'nt work?