Cannon suddenly loses range

Turn X - I have 3 cannons 2 hexes out attacking a city-state. All 3 are in range and hit the city. The attack works fine this turn, just as expected.

Turn X+1 - Suddenly none of the cannons can hit the city. Their range is reduced to 1. There are no intervening units, hills, etc (the only difference seems to be that the city-state built a galleass). But if I move ONE of the cannons adjacent to the city, suddenly the other 2 are now within range and can fire on the city (but not the first one, of course, since I moved it).

What's going on here?

Cannons (like other siege units) only have sight range of 1 tile, so they require a spotter with sight on the city. On the first turn you probably had some other unit with sight on the city and it either moved or got killed, losing sight for your cannons. When you moved one of your cannons closer to the city, it got sight on the city and the other two were able to fire.

It makes no sense. Oh, I see -- the city might have moved in the interim. Ha, ha.

In any case, it seems you ought to be able to fire into a hex blind. You might be wasting a turn, but on the other hand you could get lucky.

You need to have visual in the city. Just put a spy in there so you don't have to waste a cannon

So it seems to work as intended with the available information.