It works. The English have been known to drop h sounds in various words, so it may have been used as an alternate sp. for
Joanna and stuck accordingly... would have to check that though.
Johannah I'd prn jo-hannah.
Johanna I might ask about.
My preference of these is
Joanna, but Johannah is acceptable too.
Johanna is potentially confusing... My fave though is
Johanneke, but that has a dif. sound for the j (like a y) and different ending (uh-kuh).