I have a 2013 PRS Alpha and when the system is powered up, the gantry is solid as a rock. No way to move it at all. I've changed bits hundreds of times and never lost position.
I suspect your machine needs a little TLC. When was the last time the pinions were changed and/or adjusted? They are a wear item and have to be changed every so often. If they're loose or worn, you'll have a lot more slop in the system.
Try shaking the gantry with the power on and the reset button pressed. At that point, the motors are energized and should lock the gantry solid. If you detect any movement or even a clicking sound, then there's too much slop in the system and something has to be adjusted or replaced.
ShopBot Details:
2013 PRS 96x60x12 (Centroid upgrade)
4hp Spindle
12" indexer
Aspire
Rhino
Fusion 360
Ferrari 360
Prusa MK3S+
Prusa XL multi-tool