So after a few days of farming it, my take is that the drop rate is completely fine, but I am finding issue with how the CEs are sorted, let me explain, and for that look at this map:



The way Demiatmas work is that they drop per zone, the Shark Attack Critical Engagement can only ever drop a Realgar Demiatma, and because of the way CEs are sorted, this makes Verdigris remarkably rarer than all other options, because not only it is covered by a single CE, the one CE that drops it is the rarest of them all, and actively requires players to spawn the thing by killing Inkstains, which a lot of players may not even realize they need to do.

Perhaps the best way to solve this would've been to add an additional CE to the map, perhaps at the top of the spire to the left of the Calamity Bound, but as it is, Verdigris will be a lot of player's bottleneck.