No doubt we've all heard it said that sailors used to hug the coasts and feared to sail out of sight of land, for fear of losing their bearings in fog and not finding the correct bearing to find it again.
So there is no need to invoke observers on shore, struggling to see 'tiny boats' (@Khandro) shrink from perspective effect before they could be seen to dip below the horizon. The shared experience of sailors was that the land dipped below the horizon, if you sailed too far away from it.
However, if the people in power (king, pope etc.) look out from their ivory towers, see that the world is 'self-evidently' flat and issue proclamations and decrees to this effect then those with real-life experience to the contrary just have to button their lip.
"Common knowledge" becomes that which is held by the majority - the land-lubbers, not the sailors. It's common knowledge because it is spread by word of mouth among those who will never head to see in their entire lives.
Still, it's a good enough 'working theory' to allow you to get on with your (land-based) daily life without further thought. Useless for repeatedly navigating to that city far away with the expensive spices but, hey, you can't win 'em all.