Everyone, I’m in 5.3, so I cannot fully speak on 5.5, HOWEVER, JC nailed it (I think, I need to test more):
You have to be VERY careful with your States and Keyframes. Sequencer is finicky. But it needs to be finicky because it has to be able to handle myriad cases. More than likely, most of you haven’t fully vetted your State Rules and Keyframes.
DO NOT KEEP STATE (if you’re attaching and detaching).
Keep State means “maintain all current states, (transforms AND attachments)”. Keep State is great if you want your actor to stay exactly where it is after an animation, but it WILL persist attaching itself to a parent. Keep State is really for non-attachments.
If you’re detaching, you’ll want to restore state, and make sure that the detach rules are world AND make sure your transform keys line up with the end of the attachment track (brown bar).