Sign / Banner darkens when displaying tooltips (tellraw hover text, inventory, Player UUID in chat)

What I expected to happen was...:
When hovering over the tellraw text, nothing changes in the world (that is, the sign remains the same).

What actually happened was...:
When I hovered over the tellraw text with a hoverEvent, the sign darkened.

Steps to Reproduce:
1. Place down a command block to say the tellraw command, I used:

tellraw @a {"text":"","extra":[{"text":"Hover over this text","color":"white","hoverEvent":{"action":"show_text","value":"You are hovering over this text"}}]}

2. Place down a sign
3. Activate the command block
4. Hover over the tellraw text, and look at the sign as you do so. It should darken.

I attached a video of the test.

Steps to reproduce the darkening when showing inventory tooltips:

• Place some signs with text

• Open inventory

• Move your mouse over empty slots

• Move your mouse over allocated slots

• Observe the placed signs slightly _{(read: barely visible)} darken