What happens:
In Affinity Photo v 2.0.3, open an image in Photo Persona.
Apply a Live Filter (I tested with Gaussian Blur and High Pass).
Set the Blend Mode to anything other than Normal.
Gradually reduce the opacity from 100% to 0%. The effect is lessened until the slider reaches 0%, when the effect is applied at 100% opacity.
Video illustrates the effect with Gaussian Blur, Blend Mode set to Darken:
Screen Recording 2022-12-31 at 11.57.38.mp4
What I would expect to happen: setting the filter layer's opacity to 0% should result in the in filter having no effect, as it does in Normal Blend Mode.