Luke Murray

—  In the attachments you've provided, I can see that you're using modifications
No, I wasn't (unless you count the vanilla shader). I had just created a separate instance with 21w13a to test shaders, no mods.

It appears the BlusteryDay shader checks the block atlas to check if the block texture uv is correct, to ensure it only affets leaves, etc and not also other rendertype_cutout blocks. This means that if you have a non 16x16 texture, it will offset the uv from the expected loc for 16x16, and skip the if statements. However, as of now, there is no better way to check the block than just using the texture id, which (as seen here) is rather fragile