When you run the /swing command on a 4 legged creature. The animation doesn’t play, regardless of whether it says it has played it or not. I expected it to return some sort of error or play the animation itself.
Environment
Spawn in a creature with 4 legs, such as Wolves, Cows, or Sheep.
Run /swing @e[type=(mob)] (mainhand or offhand)
It gives you the confirmation that the command ran, but no animation is played.
This is intended per the release notes: