The "Save Bug" occurs every time a user quits or terminates the game and re-logs back into the world. The items that are in the hopper, weather be the filtration slot or any of the main slots, disappear, including any items in the chest above it. This is a critical bug as it completely dismantles what you are trying to create the mod for in the first place.

I can't reproduce this by relogging with Golden Hoppers and Fabric API. Are you using any other mods?