Let's say, you went up the mountain for some reason... if you or your teammates won't attack in time, the t3 will fail. The Disappearance happens when you are out of combat. You just need to make sure you are keeping aggro till someone hits it.
https://support.arcgames.com/hc/en-us/articles/360045002874-Steam-Linking-Issues You should submit a ticket here: https://support.arcgames.com/hc/en-us/requests/new I had the same problem, after few days they unlinked my old account and I could link new one.