(Fabric Loader 0.15.6; MC 1.20.1; appleskin-1.20.1-2.5.1)
Even after reinstall of both AppleSkin and Fabric.
Looks like this:

probably unique, but's worth a try.

In the config every line says "true". What I noticed is that these grey pixels fill up the whole bar the more I sprint and jump and when the grey pixel bar is full it takes half a hunger and resets. It kinda indicates how much hunger/sat I use... I'm a bit confused.

Edit: and it doesn't matter how much sat or hunger I have, the grey pixels always stay...

The grey pixels are your exhaustion: https://minecraft.fandom.com/wiki/Hunger#Mechanics

Is saturation showing for you now?

If you press F3, what does the hunger: x sat: y line say? Does it say you have more than 0 saturation?

Do you happen to have the saturation overlay disabled in your config?

sadly no. I only see the exhaustion. But I came to a conclusion. It's probably a incompatibility with the LabyMod Fabric Plugin. I tested the mod with a normal Fabric install and it worked perfectly.
Still thank you for your time. Big fan of your mods. Have use them in the past, will still use them in the future. Thank you for AppleSkin and have a great day.