Altho they need to always have access, the sheep I have rarely touch the water unless (i) they are ewes feeding lambs, when they need the extra water for the milk production since thats mostly water (ii) the fields are icy or snowy when they cant get moisture from the grass and they are being fed dry forage.
At this time of year its really mostly evaporation that means the bucket occasionally needs topping up. Quite useful as most of our fields arent on mains water so it is nice to be able to just fill a bucket and top it up easily. Of course in winter even our mains fed fields are off mains as the pipes freeze from Nov to March (in cairngorms!)
So no, not unusual, fairly standard I think. Which is a good thing in animals I think!