I believe that there is a small chance of an adult Strider, with a baby Strider mounted on it, which is saddled, being spawned (see attachment). The adult Strider can also be saddled, but not ridden. I can't seem to saddle other baby Striders on their adult Strider, so it must've happened some way else.
Not sure yet. I will update the issue as soon as I figure out.