Wow, that's a really rare model (though hardly useful).
At some point in model generation the game rolls for a trait from one of the groups, and if it rolls Depressed trait and model has Happy trait already, the game will not add Depressed trait.
Later there's another roll that might add Depressed trait (and traits from another group too) - and this second, approx 15x less likely to succeed roll actually can add Depressed trait regardless of what traits are present already.
There's also similiar issue with Clever and Dumb pair, Athletic and Lazy pair, and Passionate and Frigid pair.
Trait generation code has two other minor issues with order of operations.
For example, at one point the game checks if the model has Extrovert trait. If so, the game removes Introvert and Shy traits. But then the game checks if the model has Introvert trait, and if so - removes Extrovert. This check will, of course, never do anything. Either Introvert was already removed and the check will not trigger, or there's no Extrovert and so no point in this operation.
The same happens with Timid and Curious. If the model is Curious the game will remove Timid. But only after that it will check if model has Timid, and if yes, it will remove Curious.
