Is Nicomenthyl chemically stable?

Yes, it is. Nicomenthyl is characterized by an exceptional oxidative stability; it is thermostable, can be processed either hot or cold in any cosmetic formulation, does not have any incompatibility with other ingredients. However, like any ester, it is stable at moderately acid pH, while it has a tendency to hydrolyze at alkaline pH (> 8.0) particularly if warmed up.