Turns out, in this case, that it (and some other odd incredibly problems that were cropping up and making my game completely unplayable) were due to a stray "System: OR" in a completely unrelated section of my script. I don't know how it got there, frankly, since I don't use the OR condition, and am well aware that it's bugged. But there it was, mocking me. (Any chance that could happen due to Construct corrupting the file when saving?)
Also interesting is the fact that I have specific checks before setting any animation about whether that animation is already playing, and those were completely bypassed. So that's doubly strange.
Anyway, problem solved, and I hope that the "OR" condition is fixed at some point.