As the title says. When a structure with bubble columns is loaded by a structure block, an air block will be placed at the bubble columns' location, but the block functions like a water source. What this means:

• Water starts flowing from them and water sources can be created
• Concrete powder will be turned to concrete by them
• Can't see the block yet its shadows are there 
• "/fill ... air" will not do anything as the game recognizes them as air blocks (/testforblock returns "Air")

The last point is especially annoying for creators, as it requires them to fill the entire area with a different block first before being able to fill it to air.

Reproducing it is simple:

1. Create a structure using bubble columns and save it using structure blocks
2. Load the structure elsewhere using structure blocks