In 20w17a, it appears that hoppers connected to furnaces do not automatically collect its smelted contents, and as such do not deposit them into chests.
Steps to replicate:
-Place down a single/double chest
-Place a hopper that goes into the side of that chest (or the back, but my setup uses the side)
-Place a furnace on top of that hopper
-Add an ingredient and fuel to the furnace (example: cobblestone and coal), and the resulting Stone is not collected by the hopper
Unable to reproduce, make sure the bottom hopper isn't powered from somewhere.