My 12th gear won’t shift (Shimano 105 Di2 7170)

Hi,

My new Shimano 105 Di2 groupset won’t shift into the 12th gear (the smallest cog).

The only way I’ve managed to get it to shift is by setting the derailleur to +15 in the E-Tube app, but that creates huge issues with all the other gears.

I went to my LBS, but they couldn’t find the cause.

They have: Tested another cassette, same problem Checked the derailleur alignment and hanger alignment Removed and reinstalled the cassette

But nothing fixes it.

Any ideas?