I am also finding this frustrating, with my brand-new (and otherwise lovely) OB-X8. If I set the sustain pedal mode to "Note On", and play as I normally would, I usually end up silencing the OB-XA within moments by having 8 voices sustained and then hitting another chord while lifting off the pedal. It kills ALL the notes and there's dead air. This can't be intended behaviour, can it?
With 8 voices sustained on the pedal, playing new notes yields no further sounds. No notes are stolen from those being held. And this simply isn't how modern synths behave, not even within the Sequential / Oberheim stable: my Prophet X steals the oldest note when I play new notes over a sustained chord, and so does the Nord, the DX5 and every single other keyboard in the studio. So... not buying that this is just how synths do sustain, I'm afraid.
The other sustain mode – "Normal (release)" – doesn't have this issue, but it also doesn't actually sustain notes in the same way that holding them on the keyboard does: the notes die away. I get that this is vintage-authentic behaviour, but sometimes I want to sustain a chord indefinitely without my hands on the keyboard, and I can't actually do that on the OB-X8 because the alternative mode doesn't steal notes sensibly. What gives, and is there any chance of a solution?