I had two invisible houses. I knew sims were there because the plumbobs were showing up.
Unfortunately I had saved my game with them still invisible. They have been invisible for quite a few saves now as I have a lot of families to get around to. Finally I got around to them today.
I went into each lot and changed the name of the house in build mode, saved, and now they are showing.
Sims 3 players might remember there was a similar bug in Sims 3 or at least in the early days when I was playing it. The houses would either disappear from the map view or they would have a strange look about them. Going into the house and changing something on the lot and then saving seemed to fix it. It is possibly the same to fix it in Sims 4.
PS I am on a Mac so it may be different on a PC.
