If Brint and Brienne were two separate people (in lore) than I would be more than fine with them have such differentiating opinions. For different upbringings based on social and gender norms create varied individuals. If I was born as a girl with same parents, and origins, I would still come out with different values.
But Brienne was Brint. She has his memories and experiences baked into her membrane. Now, her disliking Arona since she is being pushy with her advances? Fair, and it makes sense for she is not exactly the kindest or cleanest of souls.
But her being angry at Arona for her people partaiking is slave trade? Yeah, no. Suddenly going from "Eh, slavery happens" to "slavery is terrible, those poor girls." just seems desenginious. You can call it change of perspective but, to me, it paints Brint not in best light. And makes Brienne more polluted as result.
Being fine with something just because it probably will not affect you... Not the best look.