At some point recently (probably the most recent hotfix) the m4a1 is despawning much faster than intended. If a player drops an m4 at a stash typically the gun will last quite a long time on the ground before eventually despawning, but that time has drastically been reduced. This is Official servers, and proven to be true as it’s been tested multiple times.

Some other things to note: it’s only been tested on the Livonia DLC, and for 3rd person servers.
You can rule out that someone had discovered the gun and taken it because that is also not the case.
The gun despawns sometime around or before the 1 hour mark, and while nobody is within proximity.
The gun is in a worn state.

Thanks in advance for investing further.