My game version is 1.0.3.20.
I have just installed the Expedition DLC.
I thought this would bump the game to 1.1.3.5 but nope, still showing 1.0.3.20.
The offline backup game installers only have patches starting at 1.1.2.4 up to 1.1.3.5.
Running the 1.1.2.4 patch yields:
"This patch does not support your version of the game please download a new installer"

Does that mean downloading the 3.3 GB underrail installer?
I should not expect save compatibility either, right?

Yes, the game changed a lot. You can use gog galaxy.

Current patch is 1.1.4.2

To see what that means, you have to go to their website, click on version history, and changelog for that version.

It took me some time to find it myself. But the above instructions is where I was able to find it.