Seeming resource bug

Something I've noticed is that once a city's influence sphere increases about the 3rd time or so is that any wild improvements (mines, libraries, etc) made in the expanded sphere don't attach to the city anymore and the resources aren't gained.  Highlight one of these and the bit where it says "attached to [city]" is missing and if you check the city the resource won't be coming in.  I've noticed this in multiple games.  Very annoying!