What is happening:
Situations that cause you to move on the ground at a rate higher than normal walking speed causes the stamina bar to go down as if sprinting. Most notably, this happens with sliding with the enchanted ice skates, and the ice effect itself if you hit the ice going fast enough.
Occasionally, this happens when hitting the ground smoothly after normal (non-sprinting) pegasus flight. (This is understandable lore-wise, since the pony would be running fast at that point.) Though, this effect seems to function even in cases when a pegasus is in "walking mode" in the air after disengaging flight. In which case, they would just be falling.
What should be happening:
Fast actions like falling and sliding, that are not controlled by the character itself, should not cause a loss of stamina.
Steps to reproduce the issue:
The easiest way to reproduce this issue is to just wear a pair of enchanted ice skates, then slide down a steep hill. Or for pegasi, double-tap the shift key to disengage flight while not at a relative standstill.
Game client version:
64-bit Linux Mint 18.2 Cinnamon