This Free Trade treaty says that trades between us will be free, which I took to me no gold cost to setup.  However, after signing the free trade treaty with this neighbor, I went to the trade screen and tried to trade for Lead:

It clearly shows that the trade still costs full price.  It does not appear to be a simple tooltip bug either, as when I click "Buy" my treasury is docked the 42 gold.

I also tried waiting for a turn to see if that fixed it, but to no avail.

Save file here: https://drive.google.com/file/d/139e3kHa87-I_XWBCyAwRAkGlsb2tKfG3/view?usp=sharing