It's come up sporadically on the board, yes. Your correct baggage allowance is zero. The "processing" email probably isn't going to help, as it's neither the receipt nor presented during the purchase flow. However, take a screenshot of it showing the first bag free during checkout, and look to see if there's any explanatory text related to being a credit card holder.

Assuming there's not any, you can try your luck with the check-in agents, but expect to have to pay the fee. After travel, you can request a refund, which will likely not be granted, and then file a DOT complaint. The DOT rules are simple: both the price displayed during the purchase process and the receipt have to be correct. I can't promise, but I'd expect that UA would refund the bag free quickly rather than try to press a case that looks very much like bait and switch.