I've run regular grade fuel a few times with no ill effects, and actually my LT likes running mid-grade as good if not better than premium, go figure. Same fuel economy.
Does sound like a crapy tankful of regular.
I have a small gas station up in the mountains near where I live and they sell very little premium fuel and have gotten 3 bad tankfuls from them (water) in the bottom of the tank for my dirt bike and once in my Ducati ST4s.
I try to stay away from that station! As I NEVER fuel up there with the big Beemer...
Regular and premium have virtually identical energy content so you should see no difference in fuel economy on a bike like the LT which has no knock sensor. No knock sensor means the engine can't make any accommodation for octane.
Regular in my area is 87 and I have run that a few times, but would get occasional light pinging under load so I quit buying 87. I think my bike calls for 89, but I forget if that is using (R+M)/2 or just the research method or motor method. So, I generally buy premium to be on the safe side. In my area this is typically 91, but occasionally is 93. About the only engine failures I have read about here were due to detonation damage and they are bad failures. I am not going to risk that.
I wish my LT would drink anything like my brother-in-law's Wing will, but the reality is it won't. Two years ago, we had to fuel up in Newfoundland and the station only had 85 available. Granted we were at nearly sea level, but that still was inadequate for my LT. It would ping noticeably under load in high gear. I had to run in 4th to keep the ping monster at bay. The Gold Wing was happy as a clam on that cheap stuff. I should say low grade as it was far from cheap!
Without a knock sensor to allow the engine to protect itself from low octane fuel, you take a real and very expensive risk using fuel below what BMW recommends. And remember that many engines require higher octane as the miles accumulate. The LT engine runs too rich much of the time. Run your finger around the exhaust pipe of your LT and your car and notice the difference. And follow an LT when the throttle is whacked wide open. It smokes like crazy. As carbon accumulates on the piston and head, this raises the compression ratio making even higher octane necessary. So, an owner who wants long engine life will run the highest octane they can find. That way, if you can get only low grade fuel, you will have some boost when the 91+ in your tank mixes with the lower octane stuff.
Unfortunately, when I was in Newfoundland, we were already way low on fuel (less than a gallon left) so I had very little 91 octane left to "boost" the 85 I had to buy. Might have netted out to 86, but still way too low for the LT.
Your engine so you get to choose, but I think it is nuts to run less than the highest octane available in a small, high compression engine like in the LT.