I have to concur with the original complaint; it does seem to happen quite a lot in recent versions, even with the PA at max. skill, weekly testing, and automatic sending for treatment of infected girls. I've often had it with girls that have not yet shot any scenes; even photomodels whom I've purposely kept "clean".
The consequences are pretty bad given that on top of the treatment fee and opportunity cost due to unavailability (regular work & scenes), you lose the money invested when scheduling the scene. If that scene was shot to meet a work order, you also lose the associated expected rewards. Finally, there's another opportunity cost because actor unavailability doesn't free up the schedule: I can't use the studio to shoot an impromptu freelance scene the day of the canceled scene.
So from a gameplay perspective, I'd be happier with a slightly higher rate of STD occurrence, in exchange for certain girls being outright exempt from the RNG roll: virgins, those without any scenes shot, and those that are already scheduled for a scene. I assume girls on the casting list are already immune; they should probably continue to be.
You could even add male stars to that list of possible STD victims in exchange for that change. So far, I've never seen any of them afflicted by STDs (aside from MC, who has a separate mechanic and should continue to do so).