So basically there's a pretty simple exploit. If you're a count that happens to fall victim to an invasion (e.g. the begging seljuk invasion) and you happen to be de jure part of a big bad empire, you can simply swear fealty to that empire (or kingdom) and then continue on happily after being part of that big empire while your peers are brutally converted and or destroyed/replaced. This simply does not make sense. Either this shouldn't be possible at all without being independent, or if you do do it then your former host kingdom should instantly attack your new host with the old host wanting to regain your land and the new host winning very little if they succeed at that war. I don't even think a "regain land" casus belli is enough. It should be a direct "Do you want war now yes/no". Also it shouldn't be possible at all above limited crown authority, or limited in a variety of other ways. This exploit basically means if an invasion took 8 provinces and all 8 of them were human players they could instantly swear fealty to their old liege with no penalties.