Looks to me like BMW have opted for the wrong leather layout.
Showed the seat picture to my wife, who's a skilled seamstress and dress maker and has worked with leather... 'boney bum' was her first comment.
Then she discusses the way the leather is cut and the seam is longways, all the weight gets stretched into the one long panel with no real reinforcement. "What do we expect?" type comments, it will 'seat' as she termed it, and there is nothing that can really be done about it, depending on what leather quality is used and how the dyes affect the suppleness of the leather itself. Seems different colour dyes need different textures in the leather, so the possibility that some colours can be affected more than others. That was new to me.
What she also mentioned, the way the E90 seat leather was cut, the panels are across the seat and that typically spreads the load better from local pressure. The leather can yield, without the same high pressure points of the F30 seat design. A bit more to it than that, but that were here first observations.
Look at picture in the link, seems leather stretch is not at all unusual and going to be the norm.
Personally I'm not seeing any real solution to this, as other than a virtually solid base, under the leather, we'll have this issue with the seat panel design.
OK, get the dealer to swap the seat cushion, but I'd see it happening again with a few miles added, maybe a bit different, but not going to be tidy long term.
Don't like being negative about solutions, but seems BMW may have got this one wrong, by the design choice, and/or materials.