The item capability system was deactivated during Forge's port to 1.20.6 because of the new data component system.
MinecraftForge/MinecraftForge@7b782e5#diff-a6aa3c7740f816ab284f190e9121980e804de26f228e10af0ef9c4172fb0ac53
https://github.com/MinecraftForge/MinecraftForge/blob/1.21.x/src/main/java/net/minecraftforge/fluids/FluidUtil.java#L433

Wait for MinecraftForge/MinecraftForge#9981, MinecraftForge/MinecraftForge#10118 or MinecraftForge/MinecraftForge#10324 to be merged to re-add Forge to 1.20.6 and/or 1.21.