[Imperial]

Hey, I had that too (on my 2016 MCS 4dr) until yesterday when I saw that it was sticking out a bit more then usual. It was raining all night, so I though of cutting it but as soon as I gently pulled, it started to rip and come out. In the end I just pulled it out. The cloth/fabric strip was about 3 or 4 mm wide, very thin piece of something. Don't see anything sticking out from another quarter window. I have looked at the MINI instructions and there is no word about that fabric. The window is tightly attached using special sealer tape.

https://www.newtis.info/tisv2/a/en/f55-cooper-s-hat/repair-manuals/51-body-equipment/51-36-side-window-body-manual-operation/1VnY7SyhjO

I found an image online and that fabric doesn't seem to compromise the water seal