Followup from the support:
It appears that this [bug] is happening because the block editor uses a different API than the Classic Editor, and it does not register meta boxes. Instead, the metabox is registered as a React “Panel”.
Actually, we have this report on our dev team’s issue-tracking list and we’re looking into removing that panel if the “Meta Box” setting is set to “No Meta Box”.
Glad to know you got a fix in the meantime, I’ll be sharing that with the dev team as well.
Hopefully this will have an official fix soon.
