This is just two experiments.
First: With two instances of a sprite, but each instance having different frames.
Second: Two completely different sprites, each having different frames.
What I'm trying to say is, if you compare both event blocks, you can see that the first block, if the condition is satisfied, still gives me the wrong result (which is not expected). And for the second block, if the condition is satisfied, it gives me the correct result (which is expected).
The result of this experiment leads me to think that it's a C3 bug.