It seems that I cannot install the heretic pack as its listed version is 1.2.5(a) while the game and and cultist pack are 1.2.6.183. Am I missing something about this or is it simply outdated?

Nope! Its outdated. I was wondering the same thing since I installed it on Galaxy and the pack didnt seem to install at all. And looking at the version numbers it is an older build than the base game.

No way to install it at all. We have to wait for the developer to update the DLC version D:

Maybe this process could be a little bit faster if you report the problem on their discord, to the GOG-Support or whatnot but at the moment there is no other solution than to wait

26 November 2023 and still outdated!!!!

Just looked at the last update and it seems like it has been updated!

All DLCs and the base game is now on Version 1.2.7.201

YAY :D