I too have a B&M shifter in mine and it does not do it. Your shift cable needs adjusted at the base or your may have forgotten to remove the Boxster's appendix (the green plate on the bottom if a non S).
As you recall while installing there are 2 shift cables... facing forward in the car, the right one is for forward and aft movement, the other is lateral. If you did remove the appendix, then the fore/aft one needs pulled back a thread or two (or more) on the lock mechanism at the base of the shifter.
I recommend adjusting it and take your car for a drive to run thru all gears to ensure smooth shifting
before you reinstall the center console
Good luck
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