possibly the lack of fast track, and a few other issues as well.
@OP
Its probably easier to right click the animation bp and choose the re-target option.
It will create a copy of everything tied to the skeleton you choose.
You can also overwrite existing files with this process.
I agree, its not as nice as being able to change skeleton on the fly, but every animation needs to be tracked to the correct skeleton still.
I think there is/has been some work put in to remove this limitation, but the reality is unless you specifically tag things to a skeleton the engine doesn’t know how to operate the tracks.
With identical skeletons, the proper way to create a copy has been explained (even above) there’s a full topic within this section of the forum that details the step by step process.
Not sure if this supports all animation features, fast track, etc. But to make a duplicate in a pinch it has worked well enough for me in the past.
going forward I’d probably refrain from it and make hard copies 