The foot-to-ankle vertexes and neck to body have more than 1 weight, but the leg has only one, so they pull apart when you animate the foot. These vertexes in both "rings" must have just one weight.
In this pic, I have opened the N window on the right side of the 3D window and selected just one vertex. You can see where there are both RR_LOWER_LEG and R_FOOT weight in that panel. Clicking the [X] next to R_FOOT will delete the extra R_FOOT. Once this is done, the weights will no longer pull apart.
The foot-to-ankle vertexes and neck to body have more than 1 weight, but the leg has only one, so they pull apart when you animate the foot. These vertexes in both "rings" must have just one weight.
In this pic, I have opened the N window on the right side of the 3D window and selected just one vertex. You can see where there are both RR_LOWER_LEG and R_FOOT weight in that panel. Clicking the [X] next to R_FOOT will delete the extra R_FOOT. Once this is done, the weights will no longer pull apart.