Working as intended. Your borders will sometimes revive the land, creating new yield tiles for settling. Don't know the rules for it, but placing outposts (sometimes city borders too) will create a chance of yield tiles appearing over time.

Birth of summer does it faster. 

I do recall times in FE where I would see no tile yields at a spot although if I highlighted them the tile yields would appear as the information. It could be this lingering bug still floating around. But without more definite screenshots of the situation before and after, it doesn't appear that Stardock has much to go on for this "bug".

The other people are correct in saying as you change the environment you actually increase the tile yields of neighboring tiles. Flatten out some swamps with an outpost and you might see fertile land magically pop-up.