make a new void world

place a little grass, and one sapling

place two armor_stands, one inside the sapling, one not

run

/execute as @e[type=armor_stand] run execute at @s if block ~ ~ ~ air run say hi

or

/execute as @e[type=armor_stand] at @s if block ~ ~ ~ air run say hi

Expected: one armor stand says hi
Actual: either 0 or 2 armor stands say hi, based on which one of them is older (kill them and place in opposite order to see the difference)