Retargeted animation falls in ground between transitions

I have 2 sets of animations coming from different assets and I have re-targeted them to the same skeleton. When using same-packaged animations, the transition is seamless however when mixing them up, there is a weird “falling through the floor” effect that momentarily appears.

I tried several settings in the animation editors but none seemed to have been fruitful.

304433-falling-down.png

Has anyone ever encountered this?