Z offset keeps needing to be increased.

First off - I'm running duet3 hc6 board, so I'm not running RatOS. I do have the ratrig3.1 updated components installed.
Every other day I need to babystep the nozzle 0.1mm closer to the bed, to get a good first layer.
After print finishes, I save the updated value into my config.g, so if it's "G31 P25 X-28.00 Y-13.00 Z3.70" before, it becomes "G31 P25 X-28.00 Y-13.00 Z3.80".
When I run a G31, while printing, it does report the correct G31 value, so it's not as if it doesn't save, or use the value.
Anyone have an idea what could be going wrong? My logic says that at some point the nozzle should be going under the build plate...