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I've run my M5 on several tanks of non-ethanol 90 octane (US) without issue other than probably a little less power. The fuel mileage increased though. I should note that the temps were a bit cooler. It was probably in the 50-60F range at that time.
It's possible your high pressure fuel pump is showing signs of weakness. Being a 2014, you already have the updated version, but that doesn't make it bulletproof either. Dirty injectors could also factor in. Do you ever get "shadow codes" for any misfires?
It's not the greatest diagnostic path, but I would also try the octane booster for a couple of tanks and maybe get a quality injector cleaner run through. See if there is any improvement.
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