Yes, you’r correct I’m using Unreal Engine Character Rigged animation which is completly Different than standard also reported to Blender also. Here is .FBX file i used to convert in blender. Same file submitted to blender bug fixing https://developer.blender.org/T96197
– Because Auto Generated .gltf from Sketchfab is Working on your plugin. so that’s also blender’s Problem also.
– but one case senario is gltf exported from blender is working on this site : https://gltf-viewer.donmccurdy.com/ & Not working in your extension. u will get source there also.
– 1 more must needed fix is. Model is easy to download from the browsers Developer source mode. if you can fix this it will be great addon to this.