All I can think is a malfunction between the main cooling chambers and the reservoir box, in which case removing from the reservoir will result in removing from the main...
Check the radiator cap is sealing properly if not, it can puke out coolant into the overflow when hot and not draw it back in when it cools.
@marcom started
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All I can think is a malfunction between the main cooling chambers and the reservoir box, in which case removing from the reservoir will result in removing from the main...