Mixamo is key-framing the armature instead of hips

Hi everyone,

I have a baffling issue when using the Mixamo auto-rigger and then attempting to download some walking animations for the custom mesh. For reasons I can't understand, when the walk animation is downloaded, the hips (the root of the skeleton) is rigidly locked in place, and it's the rotation of the whole armature which is doing the bounce and sway of the hips. This means when I import into UE4, the character looks like an action figure, awkwardly hobbling with their hips frozen in place. I also notice that the size and position of certain bones was different to the uploaded mesh (e.g. hip bone was a lot bigger and moved up to the stomach area).

I have tried to identify the cause of the problem, but it seems completely random. I even tried uploading the exact same rigged mesh to Mixamo twice and downloading a walk animation and the first time the hips were animated correctly, and the second time it wasn't.

Is this a bug, or is there something I'm doing which is causing this problem? Any ideas on where I can start looking would be much appreciated, as I have no idea how to debug the problem.