The early morning prior to 1.1.2, I had 5 active quests in my quest log:
3 from Alik'r - Gone Missing, Trouble at the Rain Catchers, and Tharraya's Trail
1 Guild - Proving the Deed (Fighters Guild)
1 Main Quest - Valley of Blades
Gone Missing was the last quest I had and I logged out in Bergama 10 minutes before maintenance.
When I logged back in after the patch, I only had 3 quests and regained the 4th which was the main quest by the prophet appearing to me in Bergama. The 3 remaining were Gone Missing, Tharraya's Trail, and Proving the Deed.
One relog later I only had 1 quest - Gone Missing. The prophet appeared yet again to give me Valley of Blades, which brings me up to two.
I was able to regain the other quests plus one I had not formally had via quest sharing. At this point, my SO whom I had been grouped with ported to me. During the fast travel process, he lost his quests. I shared them with him in turn.
We decided to finish Tharraya's Trail, the only quest whose origin I could not remember. I can remember where to go for all the others, but if I lose that one I have no idea where to get it again. The first objective is to go to Aldunz. In entering that delve, my SO lost that quest. Entering the objective location wiped the quest from his quest log.
To recap:
Quests can be wiped from your quest log upon relog.
Quests can be wiped from your quest log during fast travel.
Quests can be wiped from your quest log when transitioning between zones, even from overworld to delve.
This renders questing impossible as for example the Tharraya's Trail quest which gets wiped from one's journal upon reaching the objective. This necessitates not only redoing all the steps one had already done but finding the quest again in the first place. Oh and then hoping that you don't lose it again in the process of trying to complete it.