Newbie looking at 6040 (China CNC) for polypropylene 15mm sheet

[JOGARA]

My stock in Fusion is the same size as my actual stock. My part is the same height as the stock so no offsets.

I am manually moving the bit down till it touches the surface. It has been more than accurate for what I am doing. Not 0.3mm out though..

I am pretty sure this is a thing in CAD though not the machine or my physical setup of it.

I wouldn't make assumptions!
Are your previous setups still working as expected?

I can test tomorrow.