A fun bug: if you are in a council or senate republic but aren't the leader, a lot of things break. A short, non-exhaustive list:

You can't promote anyone to your council
You can't declare war
You can't grant or revoke titles
etc

Basically, anything that is associated with a council/senate law is impossible. From what I can tell, the underlying cause is that the various triggers and interactions check republic_has_council_form, republic_has_senate_form, and some variables on the primary title. If you are a vassal in a senate or council republic, republic_has_council_form or republic_has_senate_form is true, but since your primary title isn't the top title in your republic, it doesn't have a senate/council and so doesn't get the various senate/council law variables.

This basically means that everything is disallowed. You can't declare war directly because your primary title doesn't have the "leaders_discretion_war_declaration" or "valid_war_targets_list" variables, and you can't propose a war declaration vote because your primary title doesn't have the "senate_vote_war_declaration" variable. And while I can understand that being disabled for some votes (maybe senate vassals shouldn't be able to declare war), not being able to appoint your own steward/... or revoke baronies seems very wrong. I'd guess that you want a few "holds_valid_merchant_republic" checks in there so that vassals of republics can still do stuff.

Also, "holds_valid_merchant_republic" checks might be useful for the various flavor events so we stop getting them in non-republic runs.