Now I experience agreeableness as sometimes a big advantage, and sometimes a moderate disadvantage...

Hypotheses that reconcile this data and anecdata:

...The relationship between prosocial tendencies and attractiveness is moderated by another variable.

I said, which was given some implicit endorsement (I think):

That deeper truth is that it is behaviors indicating high status that are attractive. Usually these are "selfish and aggressive", not showing concern with others' standards, but conspicuous vulnerability/non high-status behavior also shows high status by ignoring opportunities to display high status with selfishness and aggression. See e.g. John Mayer.