The bug

Wolves don't use their hostile appearance when they are naturally attacking mobs such as skeletons, baby turtles, llamas, sheep, and foxes without being provoked. They only use their hostile appearance when they are attacked.

To reproduce

1. Spawn a fox

   /summon fox ~ ~ ~ {NoAI:1b}
   

2. Spawn a wolf
3. Observe the wolf attacking the fox
   → The wolf does not use its hostile appearance even if they were not provoked (does not show red eyes, their tail is not straight and also does not growl)

Video

wolf appearance comparison.mp4