Using keyframes with Threshold (and other effects)

I seem to be having trouble using keyframes with certain effects.  Threshold is one of them.  I can add the Threshold effect to a video clip and when I adjust the intensity I see the effect.  As soon as I add a keyframe, the intensity setting does nothing.  I see the slider moving as I move along the timeline, but I don't see any change to the image.  Even if I simply manually adjust the slider (after having placed one or more keyframes), there is no visible effect.

Am I doing something wrong here?  I don't have this same problem with, say, the Color Wheel, or things like cropping, with keyframes.

I just tested this on two different systems, works fine. I'd start by deleting all of the render files for that Library, turn off background rendering.