Anyone else having this issue? This is relatively recent for me, and I'm convinced it's either a game issue or GOG installer issue. I've installed the game through GOG and the offline installers, but the game isn't recognizing that I own the DLC.

Me too. Bought all the DLCs and can't install anything that isn't the cultist edition on my steam deck as it straight up doesn't recognise that the other DLCs exist. Tried uninstalling and reinstalling, and using the offline installers too and that did jack all. Hella annoying