It is true, only from the point of samsara does nirvana appear as permanent since it isn't possible to revert from it. In it's own nature it is beyond permanent and impermanent since it is completely beyond time.
This brings up an extremely strange and heterodox issue, namely is nirvana really non-revertible?
Can we address this issue without going into the field of secret, very secret and the utmost secret practices & secret dharmas ?
There are some indirect views concerning it in the normal Mahayana buddhist literature. This could be a basis of approaching it. It goes against a very basic buddhist fundamentum, which is the reason why one doesn't really want to take it up for discussion. But maybe it is necessary ?
One way of seeing it is in terms of dhyana & prajna: Let's say you attain enlightenment, i.e. stream entry or the first bodhisattva bhumi, or other stages of seeing into the reality. Time goes by and gradually everything changes, if it now happens that you become ordinary again, then we can conclude that your enlightenment was in fact only a state of dhyana or a state of samadhi.
According to the sutras and the abhidharma dhyana and samadhi can easily last for a few thousand years or more. You spend this time in a mummified body on earth or alternatively you die on earth and continue your samadhi or dhyana in a heavenly world.
In Zen and general Mahayana literature there are references to that a bodhisattva enters samsara again and attains enlightenment again, for the sake of others.
If the incarnate tulkus or other nirmanakayas are enlightened, why do they need to practice dharma again? -to attain enlightenemet again ? Have they lost it in the Bardo, or even before the bardo state ?
Then there is the view according to which all religions are based on Enlightenment. Thus we have thousands and millions of people involved in the project of making each and every enlightened person an ordinary person again. After this the theistic systems will attribute the qualities of the enlightened person to their "Supreme Being". It exists and it works.
It would be silly to atribute this only to the theistic religions, buddhists also do it. Enlightenment it is regularly attained by the "wrong persons", i.e. by some people who are not accepted by the authoritarian systems that we have in buddhism. Thus the same kind of systems are applied and practiced, through which these people are utterly destroyed. Their enlightened qualities are transferred to a figure that is put there for this purpose, in various buddhist institutions.