The TAB key -

It changes the visibility of UI elements.

It also pins missions into your list, that have been recently flashed on your screen.

You already guessed what keeps happening: something flashes on the screen, FADES AWAY, I want to make my minimap visible.. aand my pinned missions are trashed now with some random nearby crap. Because short time after it has already faded, the tab still pins that thing into the list before doing anything else..