Good to know, thanks.
Seemed unlikely that you shouldn’t be able to start with open seps at all, but wasn’t sure if ‘main’ was actually supposed to behave differently from ‘stby’ or something.
Not that it makes much difference in the sim currently, given we still can’t even switch the seps to ‘off’ and debris ingestion isn’t simulated anyway…
Guess it’s time for yet another bug report, for whatever good they do.
But hey… Hope dies last, as they say, right? 
€dit
Posted the bug report. If anyone wants to vote, here’s the report for the faulty open inertial separator startup and also the one for the startup with condition in cutoff.