Correct, I can confirm this. Sometimes tileyields just disappear and reappear.

Only tile yields with Food appear on the map.  Upgrade tile yields show when the city can pick a new tile.  Is this the issue, or something else?

The problem is that the tiles that have yields do not (always) show them. Even when they are not shown, if you know they are there, then you still can found a new city there. They disappear from both the normal world map and from the cloth map.

This is a bug that should be known by SD (and especially by Scott) since there have been multiple postings about it (example:[Bug] Beta 5 Opt-in: Disappearing fertile tiles.) and i have asked Scott (more than once) about it in the dev stream. He confirmed that the bug is on the bug list (in the dev stream of april 1 - hope it was not an aprils fool joke on us).

This is a major bug, because if you are not aware of the existence of the bug then you believe you can not create new cities, or you create sub optimal cities. This will confuse new players, and with release coming soon there will be new players.

"Food only" is not enough for deciding where to plant a new settlement. Knowing about production and essences is crucial to winning the game.

Unfortunately we won't get this in for 1.0, but a good candidate for an early patch. Seems like it might be nice to show tile yields in the upper right hand corner tile info display, whenever you're on a tile within expansion radius of a settleable tile.