Well it happens to me twice only but when i log out with mp5k (i didnt pack it to backpack...) then when we log in - it despair (on login screen i did see that i have mousin in hands - perhaps that is the cause -> mp5k in hands mousin on back equals = no mp5k ;P

and the secound issue - when you got a smersh backpack in for example hunting pack then on login screen it appers smersh on our back... for the first time i was afraid that i accidently swap them (but no its just an visual bug)

Sorry for any mistakes, hope that i help you a little, cheers :)!