Weird particles when placing lava with lava bucket in a lava flow

What I expected to happen was...:
To fill the lava flow with more lava

What actually happened was...:
The sound of a stone broken played, and particles of lava appears

Steps to Reproduce:
1. Dig a 3x3 hole in the ground
2. Place lava in the center
3. Wait to the lava fill the dig
4. Try to place lava with a lava bucket in the corners
5. Listen and see.