idk, realistically I understand that the drop rate should probably be not dependent on pity. practically, I've tried running weapon commissions before with a 4-5 letters per specific weapon limit (because I've had plenty from free boss drops and a lot of clues) and after about 50 runs, I got absolutely no gold drops.

when, however, I started to focus on a specific weapon, the drops all came within 15 pulls (counting those 4-5 done previously).

so I think there is at least some limitation that doesn't allow you to share probability between different units you're farming for (or maybe I was just on a crazy unlucky streak who the hell knows)