Discovered while looking about
MDL-30340, it seems that the blocks created at system level to be spread over all subcontext somehow are preventing the subcontexts (coursecats, courses, activities), to add their own options when the block is edited locally.
Simple use case: create one system-wide block to display one HTML block in all forums in a site.
It works for coursecat-wide blocks and course-blocks, but not for system-wide ones.
And used to work (20_STABLE)