Collapsed Radiator Hose

[CBRRICH]

Recently I did a timing belt, Water Pump, and Thermostat. Car has been running fine, Temperature gage right in the middle.  Shortly after, I noticed on cooldown the top radiator hose would collapse. I topped it up multiple times to eliminate as much air as possible. I noticed the bottle is still at the "Full" line where it was when I filled the system and has not moved up or down (when cold) suggesting that not much movement of coolant there. 

I bought a new OEM radiator cap for the radiator, still doing it. I disconnected the overflow line from the radiator filler neck to eliminate the possibility of a plug in the overflow pipe, no change. Collapsing hose, new radiator cap, water pump and Tstat are all OEM Subaru parts.

Now I'm stumped: If the radiator cap is supposed to backflow  to relieve a vacuum and return overflow coolant to the radiator why is the hose collapsing?