It happened once to me!
Same thing happens to chat icon sometimes so did not think much of it but it could be an extant dormant bug reacting at certain times.
Maybe the background updates causes this occurrence? But then why the new quest tab and not the others...
Still might be related with the chat icon disappearing incident that needs game reload to reappear.
1.122.54127899b3 (12.03.2018 8:22), Tiraje (96405), zz1, en_US, WIN 28,0,0,161, Windows 10, Chrome/64.0.3282.186, 1920x949, OpenGL (Baseline Extended) (336 MB VRAM)