When do splitting deck supports become a problem?
It is rather common for posts of that sort to develop cracks like that if they were cut from logs and then installed without being kiln dried. Even kiln dried timbers can develop cracks along the long grain but that will happen to a lesser extent. 

Cracks like that appear when timbers dry out in an uneven manner. They should not greatly affect the vertical support strength of the posts. You can watch them and if one post also begins to warp or twist then you would want to get it replaced.