Can you verity that the tube from the radiator cap neck to the expansion bottle cap is not loose, and actually leads all the way down to the bottom of the expansion bottle.
There are two outlets on the expansion bottle cap, one is the air vent and the other is for the pipe/tube, if the pipe is loose or leaky, use a thin cable tie to secure it as it maybe sucking air.
Air is easier to suck than liquid, if you see your radiator empty/low but coolant keeps on filling in the tank then you may have a leak too somewhere which is causing air to get in and replacing the lost coolant
The way its doing it is
When hot the coolant is expanding and lifting the radiator cap spring to eject it and get deposited in the overflow tan
When its cooling down a negative pressure/vacuum is created in the system which pulls the radiator cap hat and pulls the coolant in.
Now if you have a leak that negative pressure pulls in air from the leak site, which can be anywhere and all the way down to the expansion tank tube. Thats why I asked to check that too.
If topical naked eye issues dont exist then do a pressure test on it.
Expected points:
I hope you are not opening a warm/hot radiator cap and actually allowing the system to actually do a heat/cooldown cycle itself.
