Rsniper4

So I just did a full heater core flush and coolant flush few days ago.
Things went pretty smoothly, I bleed the system using the hole in the heater core return hose. System filled with 50/50 distilled water and G12. I have heat now and no overheating.

Next morning, my expansion tank is pretty much empty, but the ground underneath is completely dry!
I fill up the tank with distilled water to the max line and take a drive.
10 minutes later I stop to to check notice the expansion tank is boiling furiously and is dumping coolant out of the excess port. The coolant temp is normal and I've seen this before on systems that were overfilled.
Few hours the car is cool and and yet again the entire tank is empty.

The next morning, the tank is still empty.
I decide to not add any water, and to gently drive until the coolant reaches operating temp. About 5 mins later the gauge is right in the middle, I go to check the tank and see that there is now fluid in the tank, but about 1 cm below the min line.

The only thing I can think of is that there is a big bubble somewhere in a benign location that doesn't cause the engine to overheat. When the car warms up the air bubble expands and drives out coolant, and when the car cools of the bubble shrinks and sucks back in coolant from the expansion tank.

Currently I'm just juggling distilled water between each trip since I'd be losing considerable amounts of expensive G12 otherwise.
Anyone have experience with this? Any advice appreciated.