Your hunch is right, this is due to contained=ALLBUT
. There are limits to reusing an existing syntax. Though you can try to override or :syntax clear
certain elements, there comes a point where this becomes overly tedious.
If the original syntax author is still maintaining his syntax, you can discuss this, and submit patches to ease integration, or maybe even completely split off a common sub-syntax that you can then use to base yours on. If that's not the case, or the coupling is undesired, you'd better start creating your own, completely separate syntax, even if that means some duplication.