the “floor” from most of these preview windows in various blueprints can be customized or left on auto. Sometimes unreal will not detect the animation correctly and it will look out of place. or maybe the animation is wrong
if you click on the arrow near the perspective button you can adjust or set automatically the floor.
These walk animations in theory will require an foot/leg IK to keep them on the ground correctly in the game. So while having correct animations is important you can get away with this kind of stuff by having the IK logic do is job and find the correct position.
most of the times in the tutorials on internet about retargeting animations and preparing characters they forget, or possible they don’t know, that once you have added a retargeted animation blueprint to a characters, you need to create a foot/leg Ik specific for your character and replace the one from the animation blueprint.