Woodland mansion generates in ocean

I wanted to find a woodland mansion, so created a single biome (Creative mode) world of the Dark Forest biome, and made sure that 'generate structures' was on. 

I then used the '/locate' command to find a mansion, and upon arriving there found it in the middle of the ocean.

What I expected to happen was...:
I would find a woodland mansion in a woodland (i.e. on land).

What actually happened was...:
I found it in the ocean.

Steps to Reproduce:
1. Create a new world in single biome mode, and select Dark Forest
2. Use /locate command to find woodland mansion
3. Go to the woodland mansion

Note: I did try this again in a different world, and the mansion generated on land, so the bug only occurs sometimes.