Obviously this is just my opinion but I feel like this mechanic is too frustrating at the moment and more often than not it makes you feel cheated. In my last campaign they suddenly occupied Eilhart, Ubersreik, and gods-damned Altdorf, (which I'm fairly certain were not my least controlled areas but can't confirm that anymore). This prompted Boris to IMMEDIATELY capture Altdorf which I then could neither trade for nor take back by force since Middenland was my vassal. The same almost happened with Wissenland (also vassal) and Ubersreik, but I managed to get there before them. This occured just a few turns after Imperial Authority had tanked due to Hergig, Ostland(?), and Stirland all being destroyed back-to-back, so I didn't really feel like there was a lot I could have done to prevent this. Authority tanking due to states falling I can deal with and feel reasonable, but your capital+ additional settlements instantly changing hands as a result of this feels less like an interesting challenge and more like ♥♥♥♥♥♥♥♥.

Also worth pointing out that of the three times I've encountered a secession they all occur in Eilhart, Ubersreik, and Altdorf, but that may just be a coincidence.

I suppose it kind of goes against the surprising idea of a secession, but maybe there could be some sort of event that warned about an impending secession? Idk, just some thoughts, keep up it up

Thanks for the feedback. Very helpful. That's why I've left it as a sort of incomplete (beta) feature so far as I'm not sure how to round it off. Will do some more thinking.

At the moment another similar potential / alternative feature I have in the pipeline is an improved version of the Dark Elves loyalty mechanic for lords to secede.