Curious about this as I never experienced it on the ps3 version. I've completed the thieves guild quests ages ago, but every time I enter the ragged flagon he has the conversation options as if I'm only just returning from completing all four "special" jobs. He also never offered to buy the Eye of the Falmer from me after the questline was completed, yet bought all the previous special items. Vex's conversations also act like I'd never talked to her for help with Goldenglow. They both reset when I exit and reenter the Flaggon.

Why hasn't something so basic been fixed yet?

It's of no consequence and it has to do with the game engine, so no, it is not "so basic."

Modders have recognized it as a problem where "say once" flagged info records reset when loading a saved game, but after reloading, they work properly.

There are more important info-related issues that modders hope will be addressed.