If you're engaged with an enemy and you have the Parkour Talent, and want to engage another enemy in short range, do you just have to use the Parkour! Talent to spend 1 strain and be considered disengaged with enemy A, and then engaged with Enemy B (and then at short range to Enemy A), or would you first have to disengage with Enemy A, then Parkour! to Enemy A (though frankly at that point you could just engage them without spending the strain, unless there's obstacles in the way)?

I can't seem to find clarification for how that works anywhere.

I would say that you can't Parkour! while engaged with an enemy.

CRB100, " If the character is engaged with an opponent or adversary, they must perform a maneuver to disengage from that opponent before moving to any other location. "

Parkour! is moving to another location, so you have to disengage first.

Yeah, I agree. thought at first it might, but then in thinking on it, it doesn't say any exceptions to the normal disengage rules, so it'd need to be done before using parkour.