Made up is fine too. It is how idioms start. Someone was the first.
We also have sayings started by a commercial. You have them too.

I don't think your proverb means "pokerface" it's more like

Stille wateren kunnen diepe gronden or Geen slapende honden wakker maken

Quiet waters may have deep grounds

Better don't wake up a sleeping dog 🤔 no it doesn't cover the meaning...