Its not just hammers but all big melee weapons. Also happens with guns and non-weapon items like fertilizer bags. So atm, you can carry two melee weapons / guns. You can drop them without throwing by drag-and-dropping them out of your inventory. However, you will drop these additional items on the ground whenever you draw an item into hands.

Hi and thank you for your feedback!
This is something we have investigated and a fix has been scheduled. Please be patient.
Regards,
Andy

it happened yesterday with rice...
I ate some rice and the rice was in my jeans. then I picked up a .380 ammo box and the rice was taken away from my jeans. I searched around and couldn't find it, suddenly I saw that it was near to my shoes in the upper part of the inventory... Not in a slot but it was there on my shoulder?
I could move it again in the jeans and eat and it was all normal after that:-/

I have the same and more. Issue "Item attachted to inventory and deleted and come back bug".
If you have an addiotional weapon (example a pistol) in your inventory (your 4)) and THEN pick up an addition mosin to carry it in your hands, the mosin will be removed out of our hands if you leave a radius to the point where you found the mosin. Run back inside the radius, the mosin is automaticly back in your hands. Drop the MOSIN (not the pistol of the point 4)) on the radius-border, the mosin is teleported back at its spawnpoint.
Sorry, if my english is not good enought to understand it in my issue. I try my best.