If on PC and are in WebGL, try switching to Flash via the toggle button on the upper RT cornrr, just outside the game window. There's a known problem with the LTF not functioning corectly in WebGL so am hoping that reverting to Flash will help.
If using the app on mobile, try clearing the game cache.
If neither suggestion works, create
a SUPPORT TICKET:
Scroll down & choose a category to start the form.
BTW: you aren't required to actually clear every tile in the Event story mines. You only need to complete the tasks given in the quest in order to receive credit. (It's the same for the regular, non-event maps as well.) I, personally am a bit OCD about clearing everything, but thought I'd mention it.