This is not a bug. I have deliberately programmed the los buttons to disable once your opponent selects one of the buttons. Once your opponent stops los checking, your buttons will turn themselves back on.
Let me know if you are seeing any different behaviour.
The reason for this was to try and prevent los checking from freezing the game, which appeared to happen if both players clicked the los button at the same time. Not every time but often enough. I have not received any bug reports about los freezing since 6.6.2 was released so I am hopeful that disabling the buttons has solved that problem.
The disabling should not happen on your local machine. It will only effect your opponent's device. And can only be cleared by the person who initiated the los check closing out the los engine (by left clicking on the map), which will then enable the buttons on their opponent's machine.