The Light, Dark, Underdark! Unearthed Arcana includes the Tunnel Fighter Fighting Style which states:
[...] As a bonus action, you can enter a defensive stance that lasts until the start of your next turn. While in your defensive stance, you can make opportunity attacks without using your reaction, and you can use your reaction to make a melee attack against a creature that moves more than 5 feet while within your reach.
And the Arcane Archer Fighter's Grasping Arrow Arcane Shot feature states:
[...] The creature hit by the arrow takes an extra 2d6 poison damage, its speed is reduced by 10 feet, and it takes 2d6 slashing damage the first time on each turn it moves 1 foot or more without teleporting [...]
What happens if the creature in question is currently prone, and stands from prone? Does this activate the Tunnel Fighter's opportunity attack? Does this activate the 2d6 poison damage from Grasping Arrow?
Note I am aware that the following Q/A already exists:
But I was told in my answer to another question that I cannot generalize answers there to conclude that standing from prone does not count as moving. So I am asking these separately instead.
I wanted to ask the question "Does standing from prone count as moving?" but was told that "There is no reason for the question "Does X count as Y?" to have a consistent, universally applicable answer." and so have asked this instead.