Hi Portis, this doesn't seem to be that common on e89 but there is loads of the same door membrane problems showing up.
Discussed here, https://z4-forum.com/forum/viewtopic.php?t=94907
Look at this at about 6.30.
https://www.youtube.com/watch?v=IVO2AFwOV2k
You say your membrane is good but are you certain it hasn't been lifted before and not sealed back properly?
I'd say on a 2016 car your seals shouldn't perish, more likely something not refitted correctly, and you are looking for a sizeable leak.
Is the boot dry, under the battery etc? Does the roof work properly? Has the windscreen been replaced? Have you checked underneath the car?
Problem with water of course is it can pool a long way from where it enters.
On your other thread on carpet removal, it might be worth the commitment to stripping it completely, at least you can do some proper tests with a hosepipe and you can dry it completely once you crack the problem.