I have stack size set to 4 at it's working okay for rares but not for treasures.

When multiple rares have been found, they are displayed at the same time as they should but treasures are not: the last found treasure always overrides any previous ones, even if those have been found a few seconds earlier and even when they are still on the screen/minimap.
It looks like if the max stack size were set to 1 for treasures so only 1 can be displayed at a time.
Doesn't matter if the treasures are of the same type or different ones. Tested this with all TWW treasure types (Pile of Refuse, Machine Speaker's Reliquary, Arathi Treasure Hoard, Crabber Supplies, etc.).

I'm pretty sure this was working some weeks ago but I can't remember when that exactly was.

Are you sure it's happening with different types? I can't recreate that.

(Individual types are, which makes sense because each type has a distinct ID that's being used. It's possible that I should make each instance of that type count separately, of course.)

Yes I'm sure. You can reproduce this in no time in The Ringing Deeps where the different kinds of treasures spawn most often.
But multiple instances of the same type of treasure shouldn't override each other either and I'm 99% sure they did not do that in the past, like I was seeing the clickable targets for 4 instances of the exact same type of treasure at once.

Well I just ran into a case where different types didn't override each other so it's either not 100% or character specific (or maybe something else I don't know).