Beta testers seeking to play the Tailoring quests haven’t been able to continue due to a bug where the targeted mobs to loot Runic Catgut from aren’t the same mobs described in the quest. The quest asks you to obtain catgut from manasabers, but instead the quest is listing owl-grypon-like mobs.
A beta tester named Frimlin said in the forums there is an alternative to obtain Runic Catgut until the bug fix arrives in an upcoming patch. There is a rare elite named Myonix in Suramar which drops Rat Catgut. If you take the flight to Shal’aran, follow the river heading north toward Tel’anor.
He sits on the right side before the waterfall, and respawns within 20 seconds, so you can kill him a few times to complete the Tailoring quest: Runic Catgut.
I went ahead to test that, and I confirm Myonix has a 30% drop rate of catgut. I obtained 3 Runic Catgut in 10 Myonix kills.