When using the /fill command, it does not work to replace AIR inside cave systems (or computer generated air with very specific criteria? as above-ground air replaces without issue).

This problem occurs in all modes: creative/survival/spectator

 For example... same command used in 3 instances... meant to generate oak_fence replacing AIR in a ~2 ~-5 ~0 layout.

• screenshot 1 - Does not replace AIR inside the cave, only above it 
• screenshot 2 - Replaces all AIR blocks above-ground 
• screenshot 3 - Replaces all AIR blocks above AND below ground when the BELOW ground portion is excavated by the PLAYER, and not game generated 

This is a very odd problem - I think it's possible that the "state" of the AIR in caves differs from that above-ground... however I can't find a "state" for AIR in any minecraft Wiki indicating that I should be using a different state to fill caves :/ - AIR does not appear to have any states?

Assistance is much appreciated!