It seems to be a bug, there are many reports from users that say after the update to 1.15.19 they start to have a 0.3 +/- 0.1 mm offset in Z after Auto Z calibration. Snapmaker support contacted me on this - I sent them logfiles, seems they’re investigationg. Before 1.15.19 Z-offset after auto-calib was nearly spot-on for me.
See: Discussion of Snapmaker 2.0 Firmware Updates - #651 by Hauke