Your last image shows that your sewing machine plug has no earth ground. I’m not well versed in sewing machines enough to know if this is typical for sewing machines, but typically any appliance with no earth ground will insulate the user from either conductor.

You cannot count on the neutral wire to safely ground anything that the user touches.

Your readings indicate that there’s a small amount of voltage coupling onto the housing. Voltage alone doesn’t indicate much if there’s no current behind it, so it could be safe, but as another user indicated, you probably want external metal bits to be earth grounded. That’s the whole point of earth ground.