I have experienced the same, though I’ve done less visual comparisons. Comparing % quality like-for-like between webp and avif, on average avif files are slightly larger than webp counterparts.
Things didn’t materially improve when using high compression (slow) encoding settings. Could have been a bad encoder (libavif iirc)— there might be an explanation but on the face of it I saw worse performance than webp.
It could be my encoder sucks, could be I didn’t pick the right settings (command line opts look like the engineer’s console on the original 747). Most of the demos I have seen online with AVIF images are highly compressed images that look awful on close inspection. Maybe you can accept that for video but I can’t for my photographs.
I was disappointed.