I created a custom character for my game using Adobe Fuse that was provided on Steam. After porting the character model into Mixamo to get a base skeleton and then into Unreal, the skeleton appears to be fine.
Is the skeleton a 100% identical when you retarget it ? So do you use the humanoid rig and have you set up the bones properly?
The mixamo skeleton is a little bit different you have to set some bones manually.
This is caused by the twist bones, which are specific to the UE4 skeleton. Simply remove the names of the twist bones when retargeting and all will be well. Look at this questions and last answer. Mixamo to UE4 skeleton problem.