A Portable Hole may be formed of special spider silk. However, it is still a pocket dimension and certainly not invulnerable.
Common sense and logic says a cut (rift) can be just as destructive as one in a bag of holding. It does not have to do with magic but the kinetic and potential energy of the molecules inside, which would not just disappear into nothing like popping a balloon in this dimension. Einstein's \$E=MC^2\$ establishes that energy cannot be created or destroyed.
Dimensional Rifts can cause very strange quantum physic effects because they exist in a different time-space-dimension in the continuum. The possibilities could include (DM's discretion) anything from a tiny supernova, to a tiny black hole, to an opening to the vacuum of space sucking out the atmosphere, to deadly poisonous gases from an alien atmosphere, to flaming gasses, to a star's flames, to a wormhole, to a paradoxical unexpected event, up to a portal to another dimension.
A portable hole is 2-sided and open, as it can be folded or rolled up but not closed like a bag of holding. So whatever the DM decides happens will be a 360 degree area of effect and stopping it would not be easy at all.