Looseness at the drive shaft inner flange

I am trying to diagnose a steering problem on my car. A slight tendency to move left and right with the steering wheel at straight position, as if it wants to go off the straight line. Upon inspection of outer hub bearings, tie rods and bushings which are all firm, I found out that the left side only half shaft inner flange at the AT box has quite a radial looseness. It does not leak fluid, meaning the looseness has not damaged the seal. The right side flange has no free play. What gives?