It looks like there's a bug in the process of copying an existing character that causes the copy to lose any Weapon Mastery selections it has. This seems to happen for any class that has this feature. The selected weapons are not copied over and you have to select them again on the copy.
In fiddling with this I also noticed that the copy process also doesn't copy the state of the "Homebrew" checkbox on the Home tab, or the "Underdark Mode" toggle on the character sheet. Both of these are always in their default state on the copy (on and off, respectively).
It looks like there's a bug in the process of copying an existing character that causes the copy to lose any Weapon Mastery selections it has. This seems to happen for any class that has this feature. The selected weapons are not copied over and you have to select them again on the copy.
In fiddling with this I also noticed that the copy process also doesn't copy the state of the "Homebrew" checkbox on the Home tab, or the "Underdark Mode" toggle on the character sheet. Both of these are always in their default state on the copy (on and off, respectively).
pronouns: he/she/they
I spotted the same issue.
I'm testing out different chars for a Oneshot, so needed a lvl3, lvl4 and lvl5 version of all the chars.
Weapon Mastery was lost on each copy. For a Fighter both the lvl1 (3 choices) and the lvl4 (1 choice) was lost during copy