CNC manufacturers have a ton of decisions to make when deciding how to tailor their machines for their customers.  
One of those is if they choose to use a ball screw for their Z-Slider... the pitch of that screw can have various effects some of which can be seen as bad.

Two such manufacturers are Onefinity and Carbide3d.  Both are favoring speed for their newest machines.  They want to answer the call from customers asking for faster carving speeds.  

Thus, they've chosen to go with steeper pitch for their Z slider's ball screws.  Think of it as they're "leaning" into faster speeds... but they are sacrificing the holding power of the Z assembly when powered off.

This results in a z carriage dropping down under the added weight a spindle can add to the machine.

This topic is discussed extensively in various places:

https://forum.onefinitycnc.com/t/spindle-z-slider-dropping-down-on-poweroff-estop-elite-series

and

https://community.carbide3d.com/t/vfd-spindle-dropping-down-when-turned-off/

Some folks have even come up rather ingenious ideas to solve the dropping:

https://forum.onefinitycnc.com/t/z-axis-counter-balance-for-the-journeyman-elite-2-2kw-spindle/

Needless to say... it is a common "problem" with CNC machines favoring to answer the "we want faster" question.

It is largely discussed with Onefinity's Z20 slider, although we have observed it with Carbide 3d's Shapeoko as seen here.