Old SchooL BMW's remembered!
This was a regular occurance in the older days of BMW, dropping bags at speed. Many strapped their bags on and you still see this today on some 70's, etc Beemers. I had yet to hear a much newer BMW having this issue, so you're a first for me. I still have my R100/7 with original bags and one of mine passed me on the freeway one day, long ago:(. I looked over in shock as I realized its my saddlebag, sliding so gracefully alongside me at 70mph! It made quite a noise as it scraped the pavement at speed. It did not tumble, just slide along on its lid. Hardly any worse for the wear, its still on my bike with some mods by me to keep it from happening again. I bolted mine on, using a carriage bolt and wing nut arrangement, all hidden from view of course. You may find a similar solution, with some hard looking around the bag parts to see where a bolt could go. Mine come off simply by removing the wing nut from the innards of the bag(s) and unlocking the latches, that have remained in place. The latches are not trustworthy, as you may know now:) Mine bags have been this way for 20+ years now, without fail. :usa Randy13233