Yes you need to update PSM if you like to use it on Win 10, just to be sure it's working properly in any case. But not sure if it resolves your current issue with the character 255.

If I use Ascii 255 here in a PSM document I get printed "ΓΏ" on one machine but a different character on another machine, so it depends how the OS interprets the ALT+255 it seems.
It may also depend on the font used, so try with different fonts.
And also try with function CHR() in an expression for a variable: CHR(255).