I emptied my weapon, started to reload and used my heal mid reload to then pick up a different weapon immediately afterwards. The new weapon was mid reload animation which prevented me from firing, even though it had ammo in the mag.
Possible reasons:
a) I had to finish my reload animation from the weapon I had in my hand while picking up the new weapon
b) The person who the weapon I picked up belonged to was mid reload animation while dying and the picked up weapon had to finish the animation?
Either way, I don't think this is intended behavior.