Ran into this issue the other day, I'm not entirely sure why it affects one leg but not the other.
They're rigged the same way and the biggest issue is "undo" won't fix the problem!

Legs are set up with path and IK constraints (like stretchyman)
The problem occurs when you lock images and move the IK around, the mesh becomes.. 1 line entirely?
undo does not undo this effect

video of the issue:
https://gfycat.com/GiantFixedAzurevase

Nate escribió

This should be fixed in 3.6.19-beta, which we'll release very soon.

Aha, perfect Just ran into the issue another time but with different conditions.. (no locking involved this time, just.. animating)
Happy to hear you're working on it already