I've got the same thing on my Osage Longbow Don. I'm thinking it may be a characteristic of Osage wood? I thought maybe it was because mine was too green when I went to work on it, but after reading your post I'm starting to re-evlauate my situation?
I've even got staves drying up in the rafters that are doing it, even though I coated them with Polyurethane to stop the checking, some of them still have vertical splits running up and down the stave. Maybe it's just because the Osage is so tight grained and really super hard? No clue.
Let me know what works for yours, so I can do the same to mine.