This may seem a little complicated...
The rear slider is working fine but wont work as intended.
You need to understand what "basic" vs "upgrade" mode means.
Basic is absolute values. Eg Value 1000 = 90 degs, value 0 = 45 degs, and negative 1000 = 0 degs (preferable on the rear slider).
Upgrade is relative and requires return to centre spring mechanism to work properly. What relative means, is that the further you push from centre, the faster the camera tilt will travel. Once you let go, the camera will stop.
Basic works best on older remotes with rear slider (no return to centre).
Upgrade works best on new remotes (return to centre).
Now the problem is that the (older) controller you have is using the rear potentiometer (pot) and slider. This pot is overly sensitive as it has only 10mm-15mm of travel and no return to centre function.
Your best bet is to get the newer version controller with the built in lipo and shoulder mounted spring loaded tilt dial.
Your other alternative is recalibrate the X1 slider in BOTH the RC12 and Phantom Assist. Then nominate basic mode and move the slider delicately to get the correct position.
Also check in P2 Phantom assist > Advanced > Gimbal.
Make sure gain = 5, Vert = 0, Horizontal = 0
And finally, there is a possibility that the rear pot has been maxed to 1 direction and only allows 1 direction of travel. If this is the case, you need to open up the controller and re-centre the pot (use a Phillips head, turn max to 1 direction, then turn the opposite direction and count the turns. Half this and you will find the centre. Recalibrate again).