During the two long trips I have taken in my M3, I had an issue happened under similar circumstances. On the first trip in March, my wife complained that her seat was getting hot. We checked the temperature and the seat warmers. Temp was set to 68 (Sync and Auto) and the seat warmer was off. I felt the seat from behind and at the bottom there is a piece of leather which was very hot and I could feel heat radiating from what felt like the floor. We turned the temperature down to 62 and it started putting out cold air in the front which helped a bit. However, a little later my daughters complained that they were very hot in the back seat. I felt the air from the center vents in the back and they were blowing extremely hot air even with the temp set to 62. So I turned off the rear vents from the climate controls. But if I felt the back of the passenger seat, it was still very hot and my wife said it was still uncomfortably warm. My feet also felt warm, but I could not isolate where the heat was coming from. Outside temps were about 35 degrees and we were on software version 2019.5.15. During the rest of the trip, we observed the same behavior several more times, and some chocolate we had on the floor behind the driver seat melted completely.
On the second trip the same thing happened. No one in the back seat this time. But the seat was very warm in the back near the floor and I could feel heat radiating from what felt like the floor. Temps outside were in the high 30s. This time, I turned off the heat completely for 20 minutes at a time to try and control the temps. I also created several bug reports during the trip to try and capture data that could be helpful. Software version was 2019.8.5.
I made an appointment at the service center and talked with the tech. We played around with the controls and found that if we cranked up the heat to Hi and turned the blower up to max we could feel hot air, very hot air, coming from below the seats. There is apparently a vent below the front row seats. Since I knew that the air in the center console was blowing very hot air during these episodes, we surmised that the vents below the seats were doing the same thing, but were not turning off when I manually turned off the rear climate on the first trip. And when the rear settings were automatically off during the second trip when no one was in the back is also was blowing. After an hour or so of this very hot air blowing continuously, it could easily heat up the seat. He mentioned, and I had read about, a firmware issue that was causing hot air to blow in the back and cold air in the front. We both agreed that it was never appropriate for the difference in air temperature to be so severe when the temperature was set to sync (ie 68 for the whole car). I told him I had been recently upgraded to 2019.12.1.2. He said that it should be corrected by now and to return right away if the behavior repeated. Since it is much warmer now where I am, I might not get to those temperatures until the fall. At least I have reported the issue in case it resurfaces.
Anyone else have similar experiences? Any on a later version that 2019.8.5?