Update:
The support team has become aware of the bug.
This bug is now on the list of known issues to be fixed, I recommend that all players who are experiencing this bug continue to send their bug reports to support in order to help the team find a solution.
I've been facing this issue on both Xbox and PC, and I have friends who are playing on Steam and are facing the same issue.
I'll try to explain it in a simple way:
Basically, the trigger to start the "Finer Things in Battle" sidequest is to defeat the Ogre Thallas and then Speak with the Apothecarist during the An Untimely End.
The bug most likely happens if the player has upgraded weapons and armor. In my case, I explored the entire Map and only then proceeded to the main quest, and this bug has affected my progression by 100%;
The issue is that this sidequest is required for the "Pentiment" achievement, so the developers have a few options: Fix this bug and somehow release an update for those who haven't started the sidequest, or simply remove the "Finer Things in Battle" sidequest from the requirements for the "Pentiment" achievement, that would be the easiest and quickest solution.
I'm still waiting for a resolution to this issue, I've done everything possible on the map and I need them to fix it so I can progress in my game. In the meantime I'll play Starfield, but I'm looking forward to returning to Avowed and continuing to support Obidian.