Since 5.2 and in 5.3 when you replace skeleton on an animation (for another skeletal mesh with the same skeleton) the animation looks monstruously deformed (sometimes not in the animation asset but in sequencer).
This can look like a retarget issue (maybe something to do with the IKs?) but it is easily fixed just placing a key in the animation outline. This simple trick reverts the skeleton to the normal state.
FIX:
I’m working with many animations and this issue doesn’t happens always. I haven’t found a patron. I guess it could be something related with the IK retargeter which has not propertly conected the IKs…