The bug

Trusted foxes in version 1.14.4 does not defend the player from mobs that uses melee/projectile attacks but attacks mobs that uses a bow and arrow. I let an endermite hit me while the trusted fox was near, the fox didn't growl or chase and attack the endermite. I tried to let a mob attack me by getting shot by an arrow (skeletons, strays, illusioners, pillagers and wither skeletons with bows.) and the fox growled at them and chases the skeleton. As of the snapshots for 1.15, they also ignore bees stinging the player they trust.

How to reproduce

1. Summon trusted fox

   /summon fox ~ ~1 ~ {TrustedUUIDs:[{L:0L,M:0L}]}
   

1. Spawn a skeleton
2. Go to survival and let them shoot you then the fox growls
3. Spawn a mob that uses melee
4. Go to survival again, and the fox is not growling

Mobs that foxes attack

• Skeletons
• Strays
• Wither skeletons (Armed with a bow)
• Illusioners
• Pillagers
• Players (Multiplayer; modifiying a fox's TrustedUUIDs tag so that it defends mobs from players.)

Getting hit with an arrow from one of the mobs that shoots arrows aggro trusted foxes. It is also impossible for a trusted fox to attack an exploding creeper, as the creeper kills itself by exploding.

Foxes may ignore the following (Except for arrows and creepers)

• Melee attacks
• Ravager roars
• Llama/Trader llama spit
• Ghast/blaze fireballs
• Evoker fangs
• Splash potions (from witches/other players)
• Shulker bullets
• Snowballs/eggs (Thrown by other players/snow golems)
• Tridents (thrown by drowned/players)
• Thorns enchantment
• Pufferfish/guardian thorns
• Bee stings

Out of all of these various mob attacks, foxes seems to generally prioritize of defending the player from mobs that shoots arrows. The cause of this issue may have existed after the fix that causes trusted foxes to attack self-injured players. There is no confirmation about foxes not defending mobs that does not shoot arrows is an intended behavior. No changes were mentioned in the minecraft.net nor tweets by the developers that foxes only defends arrow-shooting mobs instead of all mobs.