Constant Interpolation on Material Curve doesn't work right/interpolates inbetween frames

Hey, so I’ve been experimenting with animation curves for animating materials (in this case controlling a character’s face textures), but I’ve found that I can’t find a way to stop the values from interpolating in between frames. The way the material for the character’s face works is that it takes a variable and uses it as a coordinate to know which face to display, so ideally, I’d just instantly set the values using the animation curve system from one key frame to the next. However, instead of instantly jumping from one value to the other like what I want, it interpolates between the two, even when the two key frames are right next to each other or when I’m using the “Constant” curve type, as you can see in the video.

Is this feature broken or am I doing something wrong?

This might be a late response, but for anyone encountering this issue:

Try changing the curve compression settings from ACLAnimCurveCompresssionSettings to DefaultAnimCurveCompressionSettings. You can find this option in the Asset Details inside your animation sequence.

I ran into the same problem in UE 5.6 and checked the curve compression settings after noticing problems with bone compression earlier. For some reason, ACL compression seems to be broken for me, and it appears to be the default in newer versions.