Why is this? Seems preposterous to me. I can't sell gold bars in the port my hideout is in despite the fact that I produce no gold bars in my hideout.. this seems nonsensical and unfair in some aspects, but I may be missing something.

Do you have a goldsmith? I am not sure what the game checks (hideout or goldsmith). Probably it checks for hideout. We will check this and if it is a hideout, we will see if we can change this to Goldsmith in a reasonable way. Goldsmith is the key part, obviously, as produced Gold Bars should need to be transported before sold for gameplay reasons.

As far as I am aware game checks goldsmith because before I built mine I was able to sell gbs in my hideout port with nonissues

I'm almost positive that he has a Goldsmith.