He seems happy enough but just to be sure does he scream if you touch his leg sensor? this is located at the top of his leg between the knee and the hip joint. If he screams when you touch that area then it could be an "injury" which will need to be treated by giving him rock salt over and over for a full battery cycle or two.
I suspect from looking at the video that he may have a loose wire or a breakage to the motion sensor in his knee joint. You could undo the skin around his foot - take off the foot plate for that leg and then roll the leg skin up to his thigh to take a look and see if you can work out what is going on. It may if you are very lucky be a loose wire that is obvious and you can just solder back into position.
Before you consider opening his skin up to take a look you need to think about his warranty. If he is still under warranty then send the video and an explanation of his issue to pleoworld or Innvo and see if they can help. If he is still under warranty and you open him up you will void your warranty.
If it helps this is what a pleo rb leg looks like
The little black tab at the knee joint is the motion sensor that tells the pleos computer which position the leg is in. There are three wires that connect up to that sensor. You may need to gently move the coloured wires to one side to be able to check where the wires join on to see if the connections are ok.
If it isn't a wire connection issue then I'm not sure what to suggest.