I was able to recreate this.

I had a repeater on my back and a blaze in my hands. I clicked the repeater and dragged it over my blaze in my hands while running. The character autoruns as he makes the switch but then you stop hearing the primary gun on you back sound. Then looking in the inventory you see nothing is on your back.

You have to look around the ground for where the gun could be.

The other day I swapped guns the same way but I must have been standing still because it did not drop anything.