Comment 7 for bug 1782797

Yes, @Seth, you are correct:

1) Select mm to edit your PCB;
2) Add the measure;
3) Save the file (doesn't need a project);
4) Close Pcbnew;
5) Reopen the file just saved;
6) Move one the the extrem or the text of the measure (always with `mm` slected);
The number will change to inch.