It was bad enough that Baha'is think Adam was a manifestation, but Jonah? Are you sure?
I am sure it is so.
There is a Book names after Him; Book of Jonah - Wikipedia
"The Book of Jonah is one of the Prophets in the Bible. It tells of a Hebrew prophet named Jonah son of Amittai who is sent by God to prophesy the destruction of Nineveh but tries to escape the divine mission.."
George Townsend who was a Christian Minister before he became a Bahai, wrote a book called the heart of the Gospels;
"...Jonah had one sign to give to the Ninevites—his inspired message of God's compassion.." George Ronald : Heart of the Gospel
The Koran Says in 4:163 (Yusuf Ali) "We have sent thee inspiration, as We sent it to Noah and the Messengers after him: we sent inspiration to Abraham, Isma'il, Isaac, Jacob and the Tribes, to Jesus, Job, Jonah, Aaron, and solomon, and to David We gave the Psalms.
The only reference I have found in the Baha'i Writings is in a Diary entry where Juliet Thompson took Abdul'baha to a Museum and Abdul'baha made a lighthearted comment on Jonah and the Whale;
"...In the Museum we passed through a room in which a huge whale hung from the ceiling. The Master looked up at it, laughed and said: "He could hold seventy Jonahs!"" (The Diary of Juliet Thompson)
Regards Tony