==This is on PC==
If you have any modified weapons for horde mode
ie. (customized anything for horde)
It will reset any weapon to said horde "preset" regardless of what the weapon looked like on the merc you are trying to edit.
This change gets made when you go to hit the customize button, doing so triggers this bug and deletes whatever the weapon looked like replacing with the horde version
Please note I have not tested to see if it does this if the weapon has no edited horde version*
This has been the case since the new customization menu was added*
When hovering over the axe in this case it shows the horde "preset" if i were to try to customize, it would apply the changes and remove the original weapon*