There are no air blocks being generated in abandoned mineshafts

Repro steps 
1. Travel to an abandoned mineshaft (/locate mineshaft)
2. Attempt a command that will check for an air block:
2a. /fill ~-2 ~-2 ~-2 ~2 ~2 ~2 flower_pot replace air
2b. /execute if block ~ ~ ~ air run say There is air at my feet!

Expected behaviour
Commands will work as normal, air blocks will be present