unwanted shadow effect (h.264)

I did see that your comp and export were the same size, but it wasn't clear if they were high resolution.

Without seeing a full size screenshot it's hard to gauge what's really going on, but it could simply be a color issue. Reds that are too saturated can cause bleeding, especially on codecs with lower color sampling like H.264 at 4:2:0.

You could bring up the lumetri scopes and see how intense that red is on the vectorscope, but as a test you could also try exporting to a codec that allows higher color sampling like ProRes 422 and see what happens. Bit rate is only part of the equation.

Exporting a solid color isn't a good encoding test. It's very easy to compress a solid color vs something with contrast or moving pixels.

The next question is what is the resolution of your comp/export, and what size are you viewing the video at when you see those artifacts? Is the screenshot above a 100% crop?

Without knowing what the original source looks like, what format it was, and what specs you're encoding to, that screenshot doesn't help much. That's not a shadow as much as it's encoding artifacts. It looks like you're either increasing the resolution on somethign that doesn't have enough detail or you're compressing it too much. Can you provide more detail about what you're doing, as well as whether you're converting a video, exporting your own graphics, and what export settings you're using? Saying MP4 only tells us the format you're using, it doesn't say anything about the resolution or bit rate.