When you say that you are "brimming it", that is not a good idea, you must leave space for expansion so only fill it up to mid way between the MAX and MIN, I have air trapped in the charge heat exchangers in my 2011 S4, these heat exchangers are situated at the highest point on the power unit, I need to jack up the rear of that car, pressurise the coolant system slightly and slightly open the 2 venting plugs. The biggest effect that trapped air is having is to make the hot coolant level be a lot higher than usual when there is no trapped air, that confused me at first as I thought that after replacing the coolant, the cooling system was self purging and so when the engine was cold, the coolant level was below MIN, what I had not realised at first was, that I was topping the coolant up to the MAX point and with trapped air in the heat exchangers, the coolant level, when the engine was hot, was causing coolant to be dumped out the top of the reservoir.
I could be that your cooling system has a failure of one of the thermostats of electric pump if it has one, and that has been causing coolant to expand more than usual in certain sections of the cooling system, now if that has happened, it could be that you also now have trapped air in some sections of your cooling system that are not able to to self purge and so, a bit like my S4 are causing coolant to be dumped out of the system when the engine is hot, you would really need to check coolant temperatures and flows at a few points in the coolant system - if there is no obvious leaks out of the cooling system.