When an entity is teleported via commands every tick, the "OnGround" tag does not update if the entity goes onto or off of a block until it stops teleporting. To test this, use these commands in the order that they appear.

/execute as @e[type=armor_stand] at @s run teleport @s ^ ^ ^0.05 ~ ~
/say @e[type=armor_stand,nbt={OnGround:1b}]
/say 1

This will teleport the armor stand forward by one block per second, so have it teleport off of a block. If this bug is present, the chat will be filled with the name of the armor stand followed by the number 1 regardless of if the armor stand is in the air or not. If the bug is gone, only the number 1 will appear when the armor stand is no longer on the ground.