I re read the Chrollo fight and its aftermath.
In one of the scenes Hisoka cleary says that he cannot form Nen around his hand any longer due to the damage, but after the fight he can reshape everything with aura and seemingly maintain its shape with Ten at virtually no cost around the damaged parts.
So Hisoka's rubber nen increased in strength because he was dead once and that strength persists even after his resurrection?
Hmm part of me doubts this because of how he phrased last command/vow. Right before he died he asked his Nen to come back to life and resuscitate him without mentioning anything about strength.
However if Nen does inherently gain strength after the user dies, then I think it would fit in quite well with what we know about Nen. Nen is all about risks and rewards based on limitations, vows, contracts etc. If someone were to make a contract for their Nen to do something after death, I feel like the Nen would remain stronger even if the user is resurrected because the contract was fulfilled. Maybe in the case of Hisoka it's that, unless his brain, lungs, and heart are destroyed, his Nen is always going to be able to resuscitate him back to life.