Sorry, there's definitively something that went wrong here. This shows the difficulty of managing forest landuse with these humongous multipolygons containing extremely precise ways.
I stumbled over the mentioned note, and loaded https://www.openstreetmap.org/relation/3105910 in JOSM to simplify.
The 'minor' simplification with an error of 25 cm wanted to remove +45000 (!) nodes, which led to many conflicts.
I thought I've fixed them all, but it seems that JOSM chocked on the uploading, as it has closed this changeset, but still hasn't uploaded all of the affected nodes...

I've reverted the whole changeset in https://www.openstreetmap.org/changeset/155179709 and redo *much* smaller extents in direct vicinity around the mentioned note.