After receiving word from the survivors of Ximenez's crew, Cortes took some ships up to Baja himself and backed up the mutinous lunatic's claim that California was indeed an island separate from the continent they had just beaten the ancient shit out of, most likely because Cortes wanted to establish a new colony he could call dibs on governing.
The Spanish government supported Cortes (as it usually did) and had a ton of maps drawn up with this hilariously flagrant error. As a result, maps all over Europe showed California as an island, including those four random and completely nonexistent lesser islands in between California and the mainland that somebody threw in there for no conceivable reason. Other explorers, such as Juan de Fuca, continued visiting California, but they kept finding inlets they didn't want to travel down. Rather than waste their time doing any actual exploring, they would all simply announce "Yep, it's totally still an island" and go on their merry way.
California kept getting copied and pasted into new maps this way for decades, until finally, in 1776, a Spanish explorer named Juan de Anza decided to literally walk from Texas to California to prove everyone wrong and correct the mistake, because it wasn't like there was anything else particularly important going on in North America at the time.