I don't have a definite reproduction for this yet, but maybe you guys might know what 1.21 change is causing this.
Since 1.21 release, we have received frequent reports of desynchronization between players in terms of the size of ammunition stacks and the amount of ammunition reported inside magazines. I encountered both of these issues after taking an MP5-k magazine and a stack of .357 out of another player's clothing. Both the magazine (15/15) and the stack (20/20) showed they were "full". Upon emptying the mag or splitting the ammo stack, you see the "true" amount. Reconnecting also "fixes" the reported quantities.
Thanks for having a look.