Anyone else's rabbits only drink water when inside?

My two bunnies live indoors but they have a huge run outside in the garden and I put them outside whenever I can. Weird thing is, even though I put a water bottle out there for them, they only ever seem to drink when they get back inside. It slightly worries me since it's been so hot outside recently. I put them in the shade but surely they need to drink too?

Anyone else experienced this?

Mine don't drink when they are on the grass as they get water from the grass.

mine very rarely drink when in their runs! they eat alot of grass so get moisture from that!