Particularly if someone is expressing a specific grievance, asking them to assume positive intent from the other party is dismissing their experience of what happened and how they should feel about it.
Sometimes people don't act with positive intent and sometimes you can fucking tell. You don't want to be telling, for example, a woman in tech who has had a coworker take credit for her work, that she should assume positive intent in that situation.
Good general guideline for self, but be careful when asking others to follow it imo.