These holes do look the right size for carpenter bees, especially if they are being excavated on the underside of the rail. Fencing isn't at great risk of breakage even if successive years of carpenter bee tunneling occurs, so you probably don't need to take any action; these bees are valued pollinators. Even secondary impacts, like easier entry into the wood for decay fungi and woodpeckers foraging for bee larvae don't pose much more of a danger to fence wood integrity compared to normal weathering and exposure to the elements.

This University of Kentucky web page details carpenter bee biology and management well: https://entomology.ca.uky.edu/ef611

If you don't want to have carpenter bees in the fence, the best times to treat wood with minimal bee kill would be late summer (after the summer emergence and before hibernation) or early spring (before nest building - they are usually searching for nest sites in April-May). As mentioned on that website, if the infestation is relatively large, then there are few ways to control them that will not kill them by either entombing them or using a pesticide.

Miri