Without going into details, just recently I was able to get pretty decent business requirements from group manager, but it seems the only reason I was able to get somewhat decent idea of what they actually do, is because there was certain level of trust since we worked together previously so there was no need to bs one another. I openly stated what I thought is doable and he seemed to understand that I need to know actual use cases.

edit: Otoh, my boss is kinda giving up on automating another group's process, because he seems to be getting a lot of 'it depends' answers.

I will say, in a lot of cases, they aren't BS-ing/lying with intent. Just the general way their minds work seemingly isn't compatible with the very idea of laying out the process in its entirety (inc. the warts/hacks/workarounds).

So what ultimately winds up happening is, you'll roll out the process according to the official way, and then it is drip-drip-drip of changes as you find out the real-world version.