Grenade swapping Automatically

Hello everybody.
A friend noticed yesterday that sometimes, when walking on and getting a type of grenade in the inventory, the grenade selector would swap automatically to the new kind even if the UNSC grenades have not been spent already.

This needs further testing actually (it occurred while playing 2 bases mode in Arena), because I want to pinpoint it very accurately and it does not occur every time.

Has anybody else witnessed something similar? Is there a topic about this already I may have missed? Or is it simply a functionality I simply overlooked?

Thanks for reading.