Hello! I tried in the past retargeting my animations in Unreal but I could not find a reliable and easy-enough way to do that. Seems like the same FBX that works for Unity does not seem to work well with Unreal. They need to be exported in a different way. I'm planning to upload the FBX versions that work better for Unreal. Also, Blender animation files are now included, so everybody can be export them in other versions of FBX or formats like gltf. I know it works good in Godot, maybe is the same case for Unreal.
looking at, i think i have a theory as to why. the bones on models have "b-" in the front as prefix. as a result, when retargeting the bones, they don't work because the names aren't the same as the default unreal model bones because they don't have the prefix. i tried seeing if i could remove the prefix on the engine, but didn't seem to work because i think it extends to the animations too. hoping that may explain the issue and how to possibly fix it.