So I did some more testing based on other comments I found online, and it seems like this bug can be circumvented by performing a second trading exchange with a merchant. After the first trade, saving is broken. After completing a second trade, saving becomes possible again. It seems like subsequent trades don't break the saving system again either. It seems like as long as the trading menu gets opened a 2nd time, that's all that is needed to fix the bug. No actual trades required, just gotta open the trade menu.
From looking at the patch notes it looks like they tried to make your save data hold onto you preference for trade vs barter. I'm guessing that's where this bug is coming from! Seems like when you perform your 1st trade after the latest patch, this bug happens.
TL;DR: Open the trading menu a 2nd time to fix the bug with saving
