This is what I do:
Rename or delete .cfg files (onWin it's %appdata%/freecad/*.cfg)
Start FreeCAD, switch to Part WB, new document, insert default cylinder. Switch to TechDraw, insert default drawing page, select the cylinder in the tree and insert 2d projection, the default and top is enough. Create a diameter dimension. Its a square on the drawing.
In property editor change the font from Sans to osifont it's a diameter symbol.
If you close FreeCAD and repeat this procedure it's still Sans. If instead after switching to TechDraw I go to Edit > Preferences > TechDraw and click on the "Apply" button (osifont is indicated as the "Label Font") exit the dialog and exit FreeCAD.
Repeat the above and the diameter symbol is shown in the drawing page, osifont is indicated in the property editor, but format spec shows
- osi.PNG (2.97 KiB) Viewed 3068 times
cut and paste yields
⌀%value%