The moment I remove any part of a leaf block (optifine, tried with shader off, internal, and several shaders) with the hammer, the entire remaining block turns invisible. Not sure what's causing this.

You need to have mipmap enabled, otherwise things like leaves and grass will be invisible.

Thanks, that fixed the visibility issue - but the leaf waving stops. Is there a way to fix that?

Do they stop after turning on mipmap for vanilla leaf blocks or after using the hammer?

What shader are you using? Does it happen with other ones?

Does the water behave in the same way?

Leaves still work with mipmap, only when I use they hammer do they stop. It happens with both of the current publicly available SEUS shaders (v11 and renewed 1.0.0), sildurs 1.21 extreme and the internal. The little tiles water block continues to hold its shader properties.

It may be a SEUS issue. All of the vanilla stained glass has a nice reflective property - but the color fades and it looks almost like the water waving visual when modified with C&B or Little Tiles. I'll get back if I figure it out.

Can you provide some screenshots?

I will close this issue for now. If it still persist just reopen it and provide some screenshots.