Not a huge issue, since my real score was recorded in the log. But it should display the correct end result, so I have something to aim for next time.

Ahh... That makes sense.
Cheers.
Although, is it documented anywhere? I can't see it anywhere in the rules or FAQ's, etc.

I believe you should not change the rule of the game;
The fact that BGA should accept solo/coop games positive score as defeats should be a feature that is implemented into BGA itself and not a hack in your implementation

I believe this is a bug, not in the technical one but in the design. There should be a text in the interface tells how the score are calculated (like Ark Nova), since the shown score are not same as the orginal game rules.

The score should correctly display the player's game results, so that it can be intuitively known by the player when sharing or when other players are checking.

Moreover, regardless of whether the player succeeds or fails, the points earned should not be ignored. Even if the display shows negative points indicating failure, they should be shown rather than simply giving a score of zero.