Partially relates to MC-147230, specifically the part when the fox dives into the water, the reason the fox doesn't try to swim is due to this issue. That is not the point of that report and is merely mentioned along with the main issue of that bug, they are separate problems.

The Bug:

Foxes will get stuck in the dive state if they don't land on roughly the same y level as their target.

Steps to Reproduce:

1.  Spawn in a fox.
2.  Spawn in a chicken.
3.  Use a lead on the chicken and fly into the air above the fox.
4.  Make sure the chicken is not too low or too high from the fox. (there is a sweet spot that will trigger the fox to dive)

Observed Behavior:

       The fox will be stuck diving on the ground and will not be able to do anything. If the chicken dies or goes out of range or either of them go to roughly the same y level, the fox will become unstuck.

Expected Behavior:

       The fox should land as normal and continue to chase the chicken after hitting the ground.

A normal player can experience this bug in 2 ways:

1. If the chicken is on a steep cliff and the fox tries to jump at it but falls short of the ledge the chicken is standing on, the fox will also get stuck if landing above their target, all of this is shown in the real world example attachment. 
2. If the chicken moves up or down from the fox in the time that it has initiated it's dive. Mainly happens when a chicken just walks up a steep hill right after the fox has started their dive.

Important Notes:

• I say roughly because if they are 1 y level apart the fox sometimes get stuck and other times does not get stuck. When 2 y levels apart it is much more consistent for the fox to get stuck.
• This works on both chickens and rabbits. Baby turtles will not reproduce this bug as the fox never tries to dive at them.
• If the fox lands on a snow layer it will faceplant in the snow like normal and then produce the Expected Behavior. (It doesn't get stuck)
• Even just one y level is enough for the fox to remain stuck in a dive as shown in the images attached. This is inconsistent, for consistent reproduction make sure they are at least 2 y levels apart when the fox lands.