My L120 had the same issue today, and more.
At one point I had the problem I think you mention. The top section of the tube simply went away. Which was great fun as the thatch I had just freed was left to spew skyward while I desperately tried to remember how to stop the blades. The lower section and the bag hanger remained in place.
Later on, while waving proudly to a neighbor from the yellow seat of my new to me toy the lower shute, upper shute, and bag hanger fell off completely. That yeilded a bit of snicker. geesh
The hanger frame has two lower tabs the just stick into a plate about the middle of the differential and the top has two hooks that mount to the Cargo Mount holes on the fenders. The hooks do not seem bent and the holes are not oblonged, so I am at a loss as to why bagger parts are auto jettisoning as well. Maybe the end of the hook needs to be longer?
I'll be curious to see if anyone has a solution, but I only need a few more hours with the bagger before the mulch plate goes on more the season until fall.
Jax