Welcome to the weak Z club.
I fought this for years…and found a way around it by
(1) lowering the Z off the stock
(2) predrilling a hole for the stock,
(3) ramping the Z down VERY slowly or
(4) lowering the Z down feed to VERY VERY VERY slow.
I cut Aluminum 95% of the time and have had almost 0 errors following those rules (In that order). 4.7 year owner…PS If you can afford it…sign up for the new Z-Plus… it will solve all the Z issues.