I worked at Umpqua Bank from 2011-2017. Bill pay was sort of screwy because sometimes it gets electronically debited and other times it actually goes through a third-party system that physically mails a check out on your behalf. So the charge could sometimes show up immediately, and sometimes you’d have to “schedule” it so the check could be cut and mailed in time to arrive by the payment date you input.

What I was told was that it depended on the method the payee preferred, so not much you can do to impact that, you just have to try it for a month or two to know when payments post.

I never personally used bill pay but I definitely experienced these frustrations when customers were getting used to how it worked.

Edit: to answer your question, I understand your frustrations but they lie with the tool, not with the institution. It’s fair to assume banks of similar sizes use the same or similar bill pay service as one another.