According to the service manual, there is no special procedure to add coolant to the expansion tank. Merely topping it off with coolant is what is recommended:
https://www.newtis.info/tisv2/a/en/i...nce/1VnYI7MiDv
It's very strange that the coolant system would be under vacuum when you released the cap on the expansion tank. Typically, a vacuum would be induced in order to bleed air off the system and then coolant added to replace air that was drawn out under vacuum. However, final adjustment to the coolant level would occur without any vacuum on the system because the expansion tank is a positive pressure device.
When you first reached out, I had assumed a leak somewhere, but after hearing that you had the sensor replacement done, it sounds like the technician did not perform the bleeding procedure properly and that there was an air pocket in the system. If there is air in the system this would draw fluid out of the expansion tank. There are auxiliary coolant pumps at several locations on this car and one of them may have been trying to fight an air pocket. When you relieved the vacuum of the system to top up the coolant, that air pocket probably wandered through the system and found it's high point in the water jacket of the top of the engine.
My assumption is that you did nothing wrong, based on the service manual link, above. I'd assume that the technician did not complete the bleeding procedure correctly or that all indications were that the system was fully bled down. A fully hot system, with the heater running, and cycling per the BMW diagnostic tool should have worked. However, I've read that others have had multiple visits to the dealer to bleed the system after receiving a high temp warning. They had major coolant system components replaced and that still didn't correct the issue. I don't know if the issue was ever resolved because the original poster never followed up on the thread:
https://bmwi.bimmerpost.com/forums/s....php?t=1494518