He does do this at times, but his omniscience tends to lean heavily towards current information, and less of a future-sight. Remember that usually, his omniscience is more like a library he can access, not a constant thing (though he does have those nasty info-dumps he can’t predict, and sometimes gets little flashes of information without reaching for it).
He also seems to have a bit of a blind spot when it comes to information regarding himself. So it would be pretty unlikely (though not impossible) for him to briefly regard the triplets before they’re born, but it’s more likely for him to randomly dissolve into a language that won’t be in use for another millennium.