Fire over a hopper is invisible, but real because it make light and hurt in survival mode. (sorry for bad english)
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Duplicate of MC-6391, please use the search function to see if your bug has already been submitted. Currently over 49% of tickets are being closed as duplicate.
I just tested, the fire is not shown, but my player did get lit on fire from it. The smoke-like particles do appear though (just not the flame part).
Recreate by placing a hopper somewhere, then crouch and use Flint and Steel on the top of the hopper.