Gaussian Blur effect heavily crops images.

I'm having an issue in Final Cut Pro where applying the Gaussian Blur effect to any image crops it heavily and I don't know why.

Take this image of a mountain:

Now, if I want to blur it nicely (say, to prepare it for the blur-sides portrait crop effect), I'd apply a Gaussian Blur effect, like so:

Now, notice how a huge amount of the sky colour seems to be missing. If we reduce the Amount to 0, the problem becomes apparent:

The blur effect has heavily cropped in the image, and it's not ideal. Does anyone know how to fix it?

Specs:

• MacBook Air, M1, 2020, with 8GB of RAM
• macOS Monterey 12.3
• Final Cut Pro 10.6.1