It's not only a problem here in the UK - also discussed on the MotorTalk forum in Germany. Translation of a post below....
"They are window seals, which are almost always spotted (after every wash or rain). I tried the following: Wax from DJ (without solvent, of course), Finish Kare - 317 Exterior Black Trim restorer, Gtechniq C4 Permanent Trim Restorer (normal applied, ie with MT after the application immediately wiped and also times 24 hours in dry to ZINO Z 16, Chemical Guys - G6 Hyper Coat, AUTOGLYM plastic and rubber cleaner ... C4 was the last rescue, unfortunately only holds up to the first wash or rain and then the seals look again stained ... Has anyone still one Idea?"
https://www.motor-talk.de/forum/fleckige-fensterdichtungen-t4012779.html#commentlistJumpMarker
"They are window seals, which are almost always spotted (after every wash or rain). I tried the following: Wax from DJ (without solvent, of course), Finish Kare - 317 Exterior Black Trim restorer, Gtechniq C4 Permanent Trim Restorer (normal applied, ie with MT after the application immediately wiped and also times 24 hours in dry to ZINO Z 16, Chemical Guys - G6 Hyper Coat, AUTOGLYM plastic and rubber cleaner ... C4 was the last rescue, unfortunately only holds up to the first wash or rain and then the seals look again stained ... Has anyone still one Idea?"
https://www.motor-talk.de/forum/fleckige-fensterdichtungen-t4012779.html#commentlistJumpMarker