This bug has existed before 8.3.8 , but it still exists in 8.3.8. If you use a Ground-type move on a Pokemon with the Sturdy ability and it fails because the Sturdy Pokemon is holding an Air Balloon, the game will claim that the Sturdy ability negated the attack rather than the Air Balloon.
The "Sturdy negated the attack" message is used when the Pokemon is targetted with a OHKO move, but it also gets used when Ground-type moves target Sturdy Pokemon with Air Balloons.
Here is what is shown:
"Ninetales used Scorching Sands!
It doesn't affect Onix...
Onix's Sturdy negated the attack!"
How to reproduce:
1) Give yourself a Pokemon that knows a Ground-type damaging move.
2) Use the NPC editor to create a NPC trainer.
3) Give the NPC trainer a Pokemon with the Sturdy ability.
4) Give that Pokemon with the Sturdy ability an Air Balloon as its held item.
5) Enter a battle with the NPC you created.
6) Use a Ground-type move on the Pokemon with the Sturdy ability and Air Balloon item.
7) Watch as the game claims that the Sturdy ability is responsible for the move failing.
I discovered this when battling Rock gyms against Pokemon like Bastiodon.
Tested using The Pixelmon Modpack on Curseforge.