I created a 2D shape animation in After Effects and added a 00FF00 background for chroma keying in 8th Wall. However, upon checking the output, I noticed a faint spill still remaining.
How should I address this issue? Does the video’s Mbps affect it?
Which sample project are you using, if any? There should be configurable parameters for the spill values. For example see the README here: Studio: Chromakey Video | 8th Wall | 8th Wall. In general though, yes the compression on the video can affect how it appears.