Outer Glow Size Reduction really choppy

Hi there! Not hyper experienced with After Effects, so simplifications will be appreciated.

I'm experiencing an issue with the Outer Glow effect in After Effects. I have a transparent rectangle where the edges are glowing using the Outer Glow effect (so essentially it looks like a glowing frame) - looks great, very happy with the aesthetic. One thing I'm experiencing though, is I'm wanting it to periodically pulse with colour, going from a bright glow and then fading to a dull one. As such I've keyframed it so that the Size and Spread of the glow alter. However, I'm finding that as the glow slowly fades, it does so in a really choppy motion, as though it goes from 100% brightness to 90% to 80% etc. without any of the fluidness in between (even though when I pause it, I can see that the keyframes are supposed hitting every 0.1% in between in the way they should normally).

I've taken a look at keyframe interpolation and tried easing in, and neither seems to have made any difference. I've also exported it and reduced quality to confirm that it's not just a playback issue. Anyone got any ideas? This feels like it should be very simple, but I can't work out what's happening. Thanks for any help!