This is making me curious too as to why the rubber strips are coming off by themselves, especially since they don't touch the bumper itself. They seemed pretty well stuck on the replacement undertray I put on.
My front bumper was replaced a few months ago due to a minor accident (someone ran an intersection). The undertray was not replaced and looked fine, but perhaps some of the plastic may have been cracked/weakened and not obvious. That, combined with the possibility that the rubber pieces may not have been in place correctly may (and I'm only making an educated guess) have caused my undertray to eventually break. This happened at a recent HPDE, hence my belief that there is significant aero/airflow going through the lower bumper and out that undertray vent at speed.
So this definitely looks like an item we should be checking on every now and then.
Golden Gate BMWCCA - Car Control Clinics
'13 C4S/'12 328i/'04 GT3
Past: '09 M3 DCT/'03 M3 SMG/'95 M3... missing an E30 I suppose...