From what I understand, the pickups (XP gems, health hearts, dollars, etc) are dropping from enemies on kill. However, I noticed that on the first map (Isolated City) there is a spot on the south-west border near the yellow bus where pickups are spawning from an unknown source, tightly grouped together in a small area (on the attached screenshot it is the pickups group with a health heart to the left from survivors). Once I pick them all up, they immediately start to spawn again, even though there are clearly no zombies at that spot, let alone any being killed. That looks like a bug to me.
There might be similar areas on other maps, but I haven't seen this on other maps yet. Neither have I seen this happening in other places in Isolated City, it's always this particular area (perhaps it is some "internal" coordinates origin, base point or something of the sort?).
I don't know how to reproduce it reliably, as I don't know what triggers it. Often I can play for more than 40 minutes in a run and never get this bug(?) triggered, and in other runs it starts to happen already in about 10 minutes from the start.
Might be unrelated or a coincidence, but once this bug(?) is triggered, SOS signals seem to always spawn at the same spot every time, which is also near that area.
Here's a video with demonstration: https://mega.nz/file/0IM3xSqJ#HtJFT96qDk_ihh4_V48CGKtBjXln60HpaZ3TR4FtipU