So it's hard to report this bug, because 30% of the times Capes work "normally", (their movement is still way overexaggerated when you walk and run) and 70% they are invisible and they make the body parts covered by it invisible.
I'd say to retrace my steps, try loading a player model on blockbench, add the cape from the "Add Parts" Tab. Export the model. Load it in game, add a Cape texture, export the model and try it out.
Thanks for the time!