Version Info

• Minecraft, 1.21.1
• Fusion, 1.2.11

What mod loader are you using?: Fabric

Are you using OptiFine: no

Description of the Bug

Grass blocks appear to use the gravel texture when transitioning on top of other blocks. See in the screenshot,
Grass -> sand = gravel
Grass -> red sand = gravel
Grass -> gravel = hard border, no transition

This works fine with just Fusion installed, so it is likely caused by some other mod you have installed.
Could you narrow down which other mods are required to reproduce the issue?

Reproduced the issue with just Fabric API, Fusion, Fusion Block Transitions and a mod called Better Babies.

It required additonal mods Data API and Gen Assets, the latter of which contained a resource pack called gen_assets.zip

I hadn't enabled it because it wasn't clear to me what it was for and I hadn't yet read Better Babies documentation. Enabling it fixes the issue.

Apologies for not being more thorough first.