well, every t6 honorific should have max socket count, that's for sure. Sounds like a bug yet I wans't able to find anything in the files. Either way I'd suggest rerolling the honorific.
Well, in cLoD I remember that when one used a too low ilvl Crystal Sword for Larzuk upgrade, it couldn't get more than 3 sockets. Perhaps the ilvl has something to do with it too in this case.
I'm guessing that since you upped it to t6 with arcane crystals it maintained the socket state that it had if that makes any sense. It's like when you uptier a unique with a crystal but no rune alongside it and end up getting a higher tier item with no upgrades. Someone should try and recreate this.