vacuum is lost

If I have this right, the door is completely surrounded by non-gas permeable tiles and should never have a route gas can enter? Or is it liquid?

In some situations, particularly when elements are being displaced or created and there are no valid spaces for them to move to in the immediate area, the elements can be teleported to the nearest candidate space - this is probably what's happening. I've personally accidentally teleported liquids as far as three tiles away, so while it needs to be relatively local to the door, it can travel a surprising distance. Closing the door works like a gas or liquid crusher and deletes the tile again, restoring the airlock.

I'd look at redesigning the area, particularly to put a bit of space between whatever is generating or displacing the element type you're finding in your door. Be mindful of things like bits of frozen gas/liquid that might be suddenly thawing and finding themselves in need of a tile to occupy - they might not be there all the time if it's an intermittent problem.

Hello and thanks for the suggestions. I took two screenshots today showing the problem (I'm playing in German...). When the vacuum "disappears" there is no other element in the automatic door and nevertheless there is a transfer of cold to the steel door - see screenshots.
I tore down the doors in question, including the surrounding area, and rebuilt them - no change. Oddly enough, I've been using this mechanism for a while and the first automatic doors are working without problems so far (about 8 pieces in total, 4 of them with problems).

A gray rectangle can be seen in the automatic door in picture 2, which happens from time to time - but there is no clear connection with the problem (which could still be the case).
I may extend the logic so that once in the cycle the door just shuts and opens no matter what the temperature is (since I'm using a single -50 degree Celsius cooling coil I've already had a few steam engines freeze when the volcano does its had a rest period).

Many greetings!

I run into the (possible) same bug. Pictures added: there are no gases or liquids in the door's frames, but the heat is still transfered.

The grey rectangles, which are part of the opening animation of the door are staying after the door is opened.

I guess the bug is that although the door looks opened, in agreement with the automation, it does not actually finish the opening sequence, and hence still transfering heat.

I'll add the "close-and-open-once-a-cycle" automatin until the bug is fixed, thanks for the idea!

For my 2 cents, I also ran into this issue.  I don't see any graphic anomalies at my door though.