Your car starts in 2nd gear; you must be talking about a 1st to 2nd gear kick down up shift. The B1 band is applied only in 2nd gear, the problem is in the B1 band or a B1 control problem.

The B1 release clearance can have too much travel, too lose. Check the part number on the piston cover, if its 126 277 25 03 then that is the self adjusting B1 piston, which some times loses its adjustment. The cover is on the passenger side of the transmission, by the bell housing. If you have the 3rd design B1 piston, I would start there first.

The B1 band coefficient of friction can be too high; the transmission could have been rebuilt with a relined B1 band, these bands don’t work will in a MB transmission. The transmission can have the wrong fluid or dirty, worn fluid.

Other possibilities are a stuck or cracked 1st to 2nd accumulator piston, in the valve body.

Do you also have a harsh 3rd to 2nd Kick down shift?