UE5.2 has a bug that makes bones unusable (no animation) when their names contain parentheses (). For example, the (mirror) function in 3DS max that can copy and flip bones. is invalid in 5.2. This problem still exists in 5.21.
The problem still exists in 5.3. Default 3Ds max mirrored bone like “bone001(mirrored)” has no animation while imported into Unreal. The only solution for now is to rename all of those mirrored bones name. 
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Bump, this issue is extremely annoying, I had to redownload 5.1 just to do some retargeting