The furnace tooltip does not show the contained inputs/outputs/fuel, and renders the arrow static and non-transparent. The screenshot below was taken of an active furnace, smelting sand to glass with coal as fuel.

Version: fabric-1.15.2-1.9.20-71

You need the addon on the server as well for that feature to work properly. What you see is typical of the addon being on the client only.

Working for me. This is one of the things I always check before pushing. Try without the other HUD mods, one of them may be leaking the GL state.