I am using two animations packs in the tutorial from the same author and do not have this issue in UE 4.27. Both packs contain Mannequin. I delete one and retarget animations to another one. Before opening, the thumbnail looks ok, but if I open the animation itself it will be shown in default posture.
Thumbnails in Content Drawer
Animation in animation editor:
Using Cover Animation Pro and Rifle Animation Pro packages. No issues in UE 4.27
I tried migrating CoverAnimsetPro and RifleAnimsetProp to UE5 without any modifications and RifleAnimsetPro works fine. I understand that it could be an issue specific to the package, but maybe there are options that I can try out?