Hi Peude,
Sorry for the misunderstanding about Mixamo support. It may be possible their character has something wrong which may be causing the issue, in which case we are not authorized to troubleshoot that particular asset.
However, we fully support the retargeting feature in UE4 and will make sure we’ve looked at every possible reason for the problem you are experiencing before deciding it is a problem with the third party character itself.
You say that when you hit “save” it returns to the default position. This is expected behavior. But if you hit “view pose” again you will see that the character is in the T-pose that you saved.
Also, when you go assign this to an animation, you should see the character in the T-pose. If it is still in the A-pose, the change was not saved and the problem will still occur.
Try this and let me know if you are still having problems.