Bug with full buckets in demo mode

Yesterday I was trying Minecraft’s demo mode. When the timer expired I had a water bucket in my inventory. After the expiration I used that bucket; placement of water was prevented (as it should be), but the bucket was emptied.

I haven’t tested that with lava buckets, probably because I didn’t have one at the moment. Unfortunately, I didn’t took any screenshot or tried to reproduce the bug once more.