Dishonored Base Game (Steam) vs Definitive Edition (Xbox Game Pass)

Gamepass games are usually a mess with the file system. It's not always compatible

IIRC one of the DLCs required you to have save game from the main game due to the way story works (they need to know whether you killed or spared Daud). Another reason to replay Dishonored.

From what I remember the DLCs (following Daud's story) have nothing to do with your progress in the base game, but the second DLC does follow on from the first.
EDIT: ...or maybe calibrationmakerarrow↑ is right ¯\_㋡_/¯ it's a looooong time since I played 😅

I've never used Game Pass but, from what I understand, the save files are completely two-way transferrable.
However, xbox uses an entirely different filing and labeling system- so for PC->XBox, you'd have to first load the game to get the folders and saves made [in your case, you'd want to clear any existing data you may have manually inserted, first], then relabel your existing PC save to match the existing xbox save file, after which you'd replace it.

Again, this is second-hand information (and even a Google search to validate the information only left me with a "well, probably.. maybe?" impression), so I can't confirm it via personal experience, but do give it a try if you haven't already. (Though it's worth noting I did find countless sources [e.g] confirming my understanding as it applied to Pass->PC, so the only reason it wouldn't work in reverse would be if xbox has a wonky system in place deliberately preventing such).

Edit: Oops, sorry; Going off the reply you made above while I was posting this, I'm guessing your situation may be more of an issue specific to the game and its difference in versions. :S Not seeing anything on Google about it, so it may not have come up before. It may be best to ask on reddit or a similar site on the matter, since it seems it'd be more likely to get focused feedback there.

These things are such a headache. Good luck.

The dlc is a standalone campaign, so you don't need to worry about the main game save :p