Again searched for this and only found this one that could be related: 8703
Could also be related to this: 9070 and this: 1899
When you are prone with the sidearm in hands, if you switch to your main weapon
your character will first stand up then go back to prone and only then switch to
main weapon.
Note that the other way around (meaning switching from main weapon to sidearm while prone) works OK.