Flavours can break down over time. This is related to things like:
Organic or synthetic flavourings - synthetics will last longer.
Temperature - the cooler a ready-to-vape product is kept, the longer it will last; although most e-liquids have a finite lifespan. (Unflavoured base has a much longer shelf life.)
The base type has an influence: PG may help to preserve flavours longer.
Many flavours lose their potency within a few months. Some have expired after 2 weeks, some last over a year.
When organic flavourings break down they may create fermentation products, and these will taste unpleasant.
We have not seen any cases of 'fungus in the lungs', with millions of posts here, so it seems unlikely.
PG, one of the base components in
e-liquid, is one of the most powerful bactericides and virucides known, when dispersed as an aerosol. This is one reason why it is used as the excipient in nebulisers used by lung transplant patients. Not sure what action it has on fungus spores though. It may be that fermentation products produced as a result of the breakdown of flavourings over time are being confused with fungus. E-liquid with expired flavours either tastes completely dead (neutral) or unpleasant.