You seem to be folding the index finger of the bow hand over the thumb. This is not a good technique because, in case of the arrow failing pieces, can be driven into the softer parts of the finger. Usually the thumb would cover the nails of the index and middle finger when using a siper. In the ordinary changal-e baz grasp without a siper the finger is folded behind the back of the bow, but with a siper you can cover the index finger tip with the thumb since the arrows travel above them.
Having the arrow cross the groove at an angle must have been common problem since I have seen diagonal wear of the leather liners on old sipers.