Reporting that I encountered the disappearing turn button in 1.80f test 2, so it's still out there It happened about half way through a gigantic game about 5 turns after the uber-pirates event. The pirates had pretty much wiped out every ship of every race still left in the game (4) and so the only ships lefts on the map were pirates.
This is not reproducable as I autosave after every turn. When I reloaded the game from my last autosave I did not encounter the bug. It seems interesting that it happened shortly after the pirate event as I thought I read somewhere it has to do something with ships/fleets thinking they still have moves left..