Is unreal using spherical interpolation in quaternion mode or is it actually doing cubic interpolation (and considering the key tangents)?
UE doesn’t support Dual Quaternions natively, however you can build a version with DQ support.
Link to corresponding thread, where you can ask AlienRenders, which interpolation method he is using:
Link to source code with DQ support:
Choose 4.22 mod version in “branch” (but at first you have to register at github)