When attaching a chorus plant to a chorus plant at the top and any of the four sides, the centre plant and the other one break.
Steps to reproduce:
1. Place a chorus plant on end stone or another chorus plant
2. Attach another plant either on top or next to it
3. Place the final plant in the other position
What I expected to happen was...
That all three would remain
What actually happened was...
The first and second plants broke, the last one placed remained
The ones that break are always the ones that are on top of each other.