Currently fluid hoppers only work on fluids in an inventory. resembling the vanilla ver, it can take fluid from the block above it plus from any other sources like cauldrons. this will significantly improve automation with the 1.17 update

Fluid hoppers will work with cauldrons as soon as LBA implements compatibility with them.

Fluid hoppers will work with cauldrons as soon as kibe implements compatibility with the Fabric Transfer API. 😝
Good news: you don't need to do anything! I hope to get a PR going this week.

would be very nice if the Fluid-Hopper could outputs some water if a renewable water source block is above it.