I personally only use millimetres in my measurements. Unfortunately my defaults seems to be easily be changed... maybe when new files are opened, I am unsure. A way to robustly lock this measurement to millimetres would be good.
Thanks for reporting this in the Ideas Portal!
With the new September 2023 Update this issue has been addressed and units of measurement for outline widths can be saved as default and will get reflected in new documents.
You can do this on Windows by clicking Tools > Save Settings as Default, and enabling the General check box.
The update can be downloaded from the Patches & Updates page.