I just started having this same problem myself on my 2009 Forester X Limited non-turbo. There is bubbling in the radiator overflow tank and the fluid never gets sucked back into the radiator like it should. After the engine cools down (overnight), when I open the radiator can I can hear a bit of a suction. When I rebuilt it in 2018, I used FelPro gaskets and seals (including HG). I've burped the radiator twice after I just replaced the water pump, thermostat and both radiator fans. I have an uneasy suspicion that my HG is failing but I wanted to see if there is anything else that could cause this. If it does turn out to be the HG, I will pull my engine again and replace them. Thank you in advance for your help on this.