The confusion you suffer when tasting something familiar but unexpected

There is no term specific to an unexpected taste. An appropriate term for the general reaction is simply:

surprise.

While à propos but unrelated term presque vu (or, "tip-of-the-tongue phenomenon") may first come to mind, jamais vu ("when a person momentarily does not recognise [something] he or she already knows") surely is more appropriate.

Restaurant reviews, e.g. of Geminis II seem to use unexpected flavor combinations and gustatory dissonance as set phrases with meanings similar to what you want, but slightly different.

Unnerving also is used, albeit not with specificity; for example, an article about fig, goat cheese, and grilled onion savory cupcakes remarks,

Have you ever thought you were going to drink Sprite, only to find that your server had mistakenly refilled your glass with water? While water can be satisfying, if caught unaware, the sip can be strange and unnerving.

Startled carries a reasonable mix of surprise and disorientation I think.