My guess is, the ankle bone in the first image overlaps with the bottom of the leg sprite and the foot sprite creating a 'joint' between the two.
In the second image, it looks like the ankle bone probably doesn't overlap the bottom of the leg sprite therefore not making a connection to that body part.
If there is a connection it may be so borderline that it's not being recognized so I would recommend moving the ankle bone up a little and seeing if it has any effect.
- TET
Animation and Video Life
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