Bolts are elastic, every fastener has an elastic limit or its yield point. When a fastener is loaded past its yield point, some of the elasticity is permanently lost. But if not torqued to yield and the load on a fastener is released, the fastener will spring back to its original length.
Our head bolts are not torque to yield, so they are not reaching the point of plastic transformation unless you are over torquing them considerably.