Temporal AA artefact/glitch

I have a scene setup with a character running towards the camera - and beyond a certain distance the character (especially the mask) is a mess of blur. After the character gets close enough to the camera, the mask pops into clarity.

Increasing the bounds of the mask helps with this, but I’m wondering what’s causing it exactly and if there are settings I can modify to improve things. It’s for use in a pre rendered video so performance isn’t an issue.