Is Action Matchbox CC broken in 2023?

Just upgraded to 2023.2.2 and the action colour correction matchbox is not working as it did before (moved from 2021.2.2).
Seems to work ok if working in ACEs but rec709 is giving some very strange results. Grading dark areas it seems to create sub black levels on the smallest adjustment, amongst other weirdness.
Is this a known bug?