Confirmed quests still there. Some freak thing with the mount that I picked up a few days ago.
Looks like one of the recent game updates fixed the problem. I am now able to add quests and achievements to the Objective Tracker, log out and log back in without them disappearing. Thanks, devs! 
it’s back, I got disconnected and when I finally reconnected after the maintenance, all my quest tracking was cleared… After spending a day fixing them back…
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Confirmed, my characters are seeing the same thing.
Relogging/logging off and back on the next time to play WoW = all my quest tracking disappears.
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Now i have the same problem, can u tell me that did u fix it ?