Gersen: Ok, I finally had time to test Dungeon of the Endless, it took much longer than expected (had to play around 3-4 hours to really test it).
TL;DR; Yes I can confirm that, when, playing offline, killing monster doesn't give you any dust. And it is something the needs to be fixed.
For a more longer explanation and for people who don't know the game, Dust is one of the resource of the game which main use is the allow you to "turn on the light" in rooms of the dungeon, you need 10 dust per room. It's an important gameplay feature because rooms with the light on can be used to build structures and also prevents monster from spawning.
The main way to get Dust is actually by exploring, when opening a room, sometime you will get a variable amount of Dust, 6, 8, 10, etc... this works the same both online and offline.
Another way if by finding it in relics / objects in the room; this also works the same both online and offline.
You also have some merchant that uses Dust as currency so you can sell your objects in exchange for more Dust.
And finally, randomly, when killing monsters you might get 1 dust per monster. This is the part that doesn't work while offline. I tried for a long time, even at the easiest difficulty, and I never had any Dust drop while offline.
Does it make the game unplayable offline ?
I would say no,
At least for what I have played I don't know at later level. At normal difficulty this drop doesn't happen very often, and it only happen on monster originally found inside rooms (i.e. monsters that were in said room when you open the door the first time), not for monsters that re-spawn or the endless spawning monsters while you run for the exit.
That said there are some object that increase the amount of dust dropped by monsters, sadly given the random nature of the game I wasn't able to obtain one of those objects and test if it really change the game or not. Also as mentioned earlier maybe in later levels the number of Dust drop per monster increase and become more important.
I was going to test this too, but it wasn't very possible for me and i got hospitalized this last week so not sure if I'll be able to test this in the short term, but given what you manage to test is easy to see that this is something we might need to ask the devs to fix.
Thanks for your service! I'll try to find a way we can all more collectively ask the devs for a fix.