Alright, thank you, Rick and MadJik. That's an easy-enough workaround for when I need it.
It's odd, because I used v3.36 and v3.5 quite a lot some years back, and I don't recall ever noticing that behavior. Maybe I just never had occasion to do an unconstrained resize, followed by a constrained resize.
Edit: Perhaps this would be too complicated or work-intensive to do, but just a thought: Would it be possible for an unconstrained resize to generate a new "original" aspect ratio, but a constrained resize not to do so?