IMhereNOW: Thanks a lot for all your replies. Here's more information on the offline backup installer patch that seems to be broken:
It's a patch for Dying Light that's suppossed to upgrade the game from version 1.49.0 to 1.49.2 and it includes an exe and bin file with the data. At first, everything seemed fine and after running the installer and letting it install for a moment it finished with "Update installed successfully". So I went ahead and also installed the 5 new bundles (DLCs) that were released (5 executables) which also installed successfully.
Then I checked the game executable and for some reason it was still on version 1.49.0 according to Properties > Details in Windows Explorer. So I started the game and in the in-game main menu it also still showed up as 1.49.0 and not 1.49.2 as it should be.
In addition to that, an in-game notification also popped up and told me that my game appears to be modified, even though I didn't install any mods. I'm pretty sure it's not my system because this happened just after I installed that official patch and those 5 new bundles. I also tried running the patch installer again and finished with "Update successfully installed" once again but the game version remains 1.49.0 and still tells me that my game appears to be modified.
I think the patch installer and maybe one of the new bundle installers are most likely broken but unfortunately, I can't inform the GOG support. As I already mentioned, the chat bot told me that it's not able to submit my ticket after I told it that I didn't use GOG Galaxy. I absolutely don't mind waiting for a fix and not playing a game for a while until it's fixed but it can't be fixed if you can't let GOG know about the issue.
Yeah. Main game files will get an update, but then they'll never update the DLC installers. I had this issue with Pathfinder for the longest time. Main game would update, so I'd install it to see what the changes were, and trying to install DLC that I paid for gave me errors because the game version was wrong. I don't know if it ever got a solution, they closed the ticket with no response. I gave up after a year.