I first noticed the issue yesterday, too.
I can confirm the issue for zz2, zz12 and zz23, for island quest icons (rewards, but not for the coin icon) and in several other windows, e.g. inventory (but funny enough not in the extended inventory, and not in the event window), unit icons in the attack and support window (but not in the troop overview of the UI), and probably in some other windows.
Happens as soon as you place the mouse cursor on the icon. You can select/click the item nevertheless, but of course that nervous blinking of the tooltips is annoying.
Additionally and only on zz2, when placing the mouse cursor on farming villages and cities, the tooltips for farming villages or city info tooltips show up only once, very shortly, and then disappear. This behaviour is certainly related to the overall issue - it´s simply one single "blink" instead of many, and seems to happen only on zz2.
My browser is the latest FF, no user scripts active.