As long as the seceding realm stays out of the war there shouldn't be a problem. Maybe they'd agree to sign a non-aggression pact.
I don't believe there should be a problem even if they stay
in the war.
It has been absolutely, 100% clear for RL
years now that this was going to happen. The timing was up in the air, but the fact of it was totally public.
That Dominic is painting this as an obviously strategic secession speaks to me of a (somewhat understandable) lack of IC/OOC separation in this case.