Panel removed.
@rotifer was correct that failure of the foam strip that runs across the panel is the root cause, but it was apparent that the panel was not removed before, the strip held firmly to the panel and the door frame but at the same time it was soaked with water as I used my hand to separate it. Interesting, I guess time or environmental conditions can cause it to deteriorate, as I got another A2 where I removed the panel and put it back without renewing this strip and there is no water leak. Also, the material seems porous, so not sure if it completely blocks the water ingress (?). I think I'll replace it with butyl rubber strips, they should hold up much better.
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