Niantic responded to my inquiry, and that is by design. Generation four evolutions won’t work for Celebi’s Special Research.
You have to evolve an already developed Grass-Type Pokémon from the Kanto, Johto or Hoenn area. Please excuse the circumstances which have been brought on. Unfortunately, Pokémon from the Sinnoh area don’t work.
(Response from Niantic help)
There have been identified points previously with Celebi’s subject analysis. Given that Grotle is Gen4 and was launched after Celebi’s subject analysis, the consensus between the 2 threads is that that is probably on account of a bug.
I reached out to Niantic with reference to this, and can replace my reply after I hear again.