Webm is just a video container, not a format. WebP uses quite outdated image compression from the VP8 video codec, which may perform quite a bit better than JPEG at very low quality, but at near-transparent quality, which images are usually encoded to, it very often doesn’t even beat JPEG.
It’s entirely an encoder issue…
That’s the dumbest thing I’ve read today
Whether 2-pass has benefits for the “constant quality” mode in AV1 encoding, depends entirely on the encoder. For aomenc, you want to always use 2-pass for slow offline encoding, as otherwise important features are disabled.
If AV1 noise synthesis “removes” banding, that banding was never part of the video in the first place, but your video player system created it during bit depth reduction, since you’re viewing on an 8-bit display. This can be prevented with dithering, which AV1 noise synthesis can substitute for.