Hello,
if you Change your Pants with hunting Pants, the old Pants despawn instantly. It dosn't metter if items are inside the pants.
And i also see it in the Video for 0.58 of "Mr. Blackout".
The only way to Keep your old Pants with stuff is first to drop the Pants and then drag and drop the new hunting Pants on your Body in the inventory. Direct Change is not working.
I've noticed this bug in 0.57. For me the old pants didn't despawn, they appeared very high in the sky. Was in one of the new schools. Default drop location has the wrong height coordinate or something.
I'm wondering has the New School structure got something to do with it? When I've encountered this bug, I've always been inside the school...? There is a horrible lag in that building