City build mode still ignores my selection sometimes

  To reproduce:

    1. Select a city.

    2. Press "b".

    3. Click a building from the list.

    4. Mouse over the map.

  Expected behavior: buildable tiles turn green, the building appears on the map, and clicking places it.

  Observed behavior: sometimes the expected behavior; sometimes the game behaves as if no building is selected, and I have to close the build menu or select another building (selecting the same building again doesn't work).  I haven't spotted a pattern in when it works and when it doesn't.