I've noticed something similar on a recent long trip in 85F+ bright sunny conditions in my X5d.

Wasn't towing, but had about 800 pounds of gear in the back. After about an hour & a half of driving on long backroads w/ moderate rolling hills going a steady 60 MPH with no stopping, power was noticeably reduced but I was getting AWESOME fuel mileage (31.5 MPG avg over about 90 miles according to the trip computer).

Pulled out the Foxwell NT520: Trans temp was 193-196F, engine oil & coolant temps were between 187-191F, boost requested vs actual only deviated by less than a PSI w/ actual boost often reading very slightly higher than requested (normal). EGT was around 600F, intake air temp was around 90F, charge air temp was around 115-120F if I remember right. Exhaust manifold pressure was nothing out of the ordinary & DPF diff pressure was around 75 mBar (normal).

Nothing I saw indicated an issue w/ any of the engine operating parameters, everything was where it should be; however, power was noticeably reduced when floored albeit w/ the absolute best fuel mileage I've experienced thus far...

Considering how perfect, smooth, & powerful the engine runs in every other scenario, & how many sensors / parts / etc have been preemptively replaced over the past few months to ensure continued performance over the next 50K miles, my best speculative guess is that the DDE is programmed to throttle power for some unknown reason under the specific conditions and / or duration I was driving. My guess would be charge air temp has a lot to do with the significant change in engine mapping.

I've driven a few 300 mile long trips on the highway w/ the cruise set to 95MPH w/ pulls to 135MPH (when safe & in Mexico of course). Zero throttling of power, engine felt as strong as it ever has; however, the ambient temps were only around 70-75F. Going normal highway speeds (75-85 MPH), engine coolant & charge air temps are measurably lower in my X5d, vs loafing around backroads going 60 MPH.