How to seal seams between wooden deck rails?
Miter joints are almost always susceptible to expansion issues like this. It's worse when exposed to direct moisture and heat. Caulking isn't going to entirely stop that from happening--may make it worse if it clogs the joint and leads to worse separation.

Some ideas:


Fabricate a metal wrap and drip ledge for the corner, if not the whole rail.
Rebuild with a different wood that's more stable.
Rebuild out of a composite material that'll never move.
Rebuild with a different joint maybe a simple butt or lap. In either case, consider sealing the end grain with epoxy or spar varnish to inhibit moisture wicking up the grain.