I tested this with the ACR and AWP. With the awp I would shoot 5/10 and then the next shot would drain it to 0 and require a reload or sometimes it would get to 1 and then jump back to 9 without reloading. With the ACR I unloaded a magazine successfully but the next one jumped from 20 to 5 and then drained normally. I just fired 8 ACR Magazines from my hotbar and I just looked down and 7 of them refilled themselves. This was all tested on singleplayer.

https://youtu.be/fByT4-MB02Q
This video was recorded directly after I had just mag dumped all of those magazines in my hotbar, I had deleted the first half of the recording because I thought that nothing had happened until I saw that all my mags were full again, couple other shenanigans seemed to happen in this video.

This one also seems to be due to a higher random tick speed on a server. I do not understand why but there seems to be a correlation.