Part in parenthesis ( ) is basically description what is in first half of the video.

(I have killed guy using m4 -with ruined suppressor, and then I have switched my half full mag to full mag in m4 by holding R button. Then I have looked what was in his inventory. I have killed zombie with FNX and put my bottle suppressor from m4 on the ground. I have tried to fill my m4 magazine but with no luck.)

1:12 - unable to fill m4 magazine (trying few times)
2:48 - 3:08 - also I think I have experienced ghost bullets (I am not hitting zombie every time but when I did there is no blood from him, he did not react to hit with knockback also).

After that ghost bullets I had m4 with full filled magazine in hands. I had left the game than connected on the same server again and I had half full magazine in m4 which I had after I killed a guy. - see images

Before reconnect: (see magazine in m4)

After reconnect: (see magazine in m4)

I suppose that killing guy, switching magazine in gun by holding R button and interacting with his inventory may cause that I cannot fill my m4 magazine with bullets.

Also I heard it from multiple top dayz streamers that you should not reload magazine in gun by holding R button, because than that magazine may became non-refillable.

We had experienced the same problem before (not on video). We had killed a guy and reloaded magazines - by holding R button, we was interacting with dead guy inventory (opening or also taking his items) and than my friend was unable to fill one of his own mags. Also my friend experienced ghost bullets on zombies. Not immediately but after shooting few zombies. Then he tried drop his magazine on the ground which was unable to fill and his game crashed.