I've had tooltips stop functioning in vendor menus, but I'm not entirely sure how to reproduce it. When it does occur, it persists through shutting down the game and reloading the save. It happens only to individual characters, not the whole party. I was using Ifan as my barterer, and I had to switch to other party members to view the items and then back to him to buy.
I'm not certain, but it might have happened after picking up a huge amount of items at once when a merchant died and I did a take all into Ifan's inventory, but I couldn't really say for sure.
The problem did seem to go away for no apparent reason as well, eventually.
