Is this siphon based recipe for mousse au chocolat trying to whip the cream? The following recipe came with my new N2O siphon:

dissolve 75g of chocolate in 275g of cream
cool to 5 degree Celsius
strain through sieve into siphon to make sure no undissolved pieces of chocolate get into siphon
pressurize siphon
shake 8-10 times

When I eat the mousse au chocolat immediately after letting it out of the siphon, it doesn't quite have the texture I expect from mousse au chocolat (my reference is a traditional recipe with eggs, whipped cream and chocolate), but it's still fluffy and nice. When I let it rest in in the fridge however, which usually gives mousse au chocolat a firmer texture without being less fluffy, it kind of deflates and reverts to it's pre-siphon state of liquid chocolate-cream mix. It actually keeps it's fluffiness worse than a vegan variant I also did with 250g of coconut milk and 100g of chocolate (which has enough fat that it becomes solid in the fridge and keeps it's fluffiness).
Is that supposed to happen like that? Isn't a whipped cream siphon supposed to, you know, whip the cream?
This is an expected outcome. Foams made with a siphon are not especially stable over time.
If you are making a siphon recipe without stabilizers, you should prepare it in time for serving, not plan to store it.