The shifter is designed to move forks internal to the transmission and not a primary method of holding it in gear.

For forward gears ( and synchronized reverse on some transmissions that may or may not be MB built ) the slider has springs / detents that hold it in position. In this case the shifter can be removed and the trans will still stay in gear.

If the shifter isn't moving far enough to fully engage a gear, that gear will pop out when loaded. A quick check for an external linkage trans is to select a gear then from under the car try to move the proper lever on the trans farther.

Sometimes shifters are worn to the point they hit internal end stops before moving the trans lever far enough.