Well, yes: Undefined behaviour in the sense the writer of the program has not defined it.

A program is written with certain assumptions about the state at certain points.  An assert can be used to explicitly state those assumptions, and halt the program (in non-release) if the assumptions are invalid.  If the state does not match what the assert assumes it to be, then any code relying on that state is invalid, and what it does has no definition given by the programmer.

(And here I've circled back to assert==assume... all because I assume what assert means)

If the state that is being checked could actually ever be valid, then it is not valid for an assert - use some other validation.