Agreed, likely the lag issue but do check which drive logic mode you are in. I have found that if you are in 4 or 5, the throttle blip when downshifting can cause a feeling that the car is surging. Especially if you are expecting some engine braking with the downshift!
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2021 X3 M40i
1974 2002tii, Inka, 5 sp manual
BMWCCA # 327475
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