hmm… So I’ve been playing with it and – if I deliberately save/close rhino and reopen, then it opens as last instance. yay!
But when computer went to sleep, rhino closed and then it opened as default (turns out this was actually the problem I was having).
I played with computer power/sleep settings (I increased it to hours before it went to sleep instead of a few min) and that seems to have resolved it but I’v not idea why or how… (new computer btw).