Many of those case are partial flown credit and that is an understandable problem, with a clear explanation and discussed How to use flight credits for partially used itinerary (cancelled during trip)?
But you stated yours was an unflown credit, so that is not your issue.

Others look like they started with the FFC, those are covered in Issues with UA's online rebooking: Missing flights, higher prices, changed cabin, .. This is against FT recommendation on using FFCs.
But you stated you waited to the payment page to apply the FFC, so that is not your issue, if we understand your situation

Yes there are issues using FFCs but none that mention the pricing changing after applying the FFC or price different if you are logged in or not, which have been the two symptoms you have described. No one disputes there are FFC issues but we need to separate out the various issue to understand your issue.
UA is not faultless but many have workarounds, once understood.

It would be more helpful to see screenshots of your situation as there are other corner cases like purchasing on AMEX travel and issues with expiration date

In your screenshots it is important to point out where you engage with the FFC.

We are not trying to be difficult but the devil is in the detials.