Hello Irbaz,
In any typical system which is fully isolated between HV and LV domains, a single point failure of the GD isolation will not create a functional issue (apart from high noise coupling). But this isolation failure would be a breach of the safety concept for human electrocution prevention.

In practice, at GD level, most isolation failures will also result in other GD failures which will be detectable. However, as recommended in the safety manual, another means of ensuring the galvanic isolation integrity must be provided at system level. This is typically handled on the battery side, with dedicated leakage measurement circuitry, to ensure the maximum allowable leakage is not exceeded. (both polarities).
BRs, Tomas