I’ve also had this problem. In my post here, steps 4 and 5 are how you would address this. The way Snapmaker appears to have implemented this, you have to use two different commands to adjust the Z-offset of the left versus the right extruders.
The problem you are experiencing is also present in the 1.15.20 firmware. I just noticed that they released 1.15.21 today, but the release notes don’t mention anything with respect to Z-offset, so I’m not expecting it to address the problem.