First, it doesn't seem like the kind of thing evolution would select for. Our brains may be susceptible to making the kind of mistake that leads one to believe in the existence of (and the need for) objective morality, but that would be a bias, nor a terminal value.

I think you're wrong here. It is possible for evolution to select for valuing objective morality, when the environment contains memes that appear to be objective morality and those memes also help increase inclusive fitness.

An alternative possibility is that we don't so much value objective morality, as disvalue arbitrariness in our preferences. This might be an evolved defense mechanism against our brains being hijacked by "harmful" memes.

Second, we can simply look at the people who've been through a transition similar to byrnema's, myself included.

I worry there's a sampling bias involved in reaching your conclusion.