Some thoughts

I think they are hung up because it would be past due if unpaid, might affect scoring, but not sure. I would think that showing the payment from your bank account would be sufficient, but technically that could be for anything, and they may want to see if you were assessed penalties maybe? I'm not calling you out, but the "variety of reasons that aren't totally relevant" seems to gloss over how you paid your taxes like 6-7 months later. From a super stringent view, there is nothing showing that most recent payment went toward 2017 taxes.

I'm assuming they got transcripts of your returns for the last couple years. Perhaps besides the recent payment, you could put together your last 3-4 years returns and match them to the bank transactions showing refunds/payments made in those amounts. If you match the amounts exactly for several years then that should lead to comfort as to the last payment being for 2017. Just an idea if you are jammed up.