My large [20+ft] cherimoya has dropped most of its leaves. I have neglected watering, so that might be a partial cause,plus some frost damage... but here are a few snips from the site below that indicate LATE spring leaf drop is normal ....

....SNIP....The cherimoya drops its leaves in late spring (or early summer) after which it blooms.

.....SNIP....The cherimoya is adaptable to Mediterranean climates. In addition to San Diego and Santa Barbara and Ventura counties in the United States,

Here is a link that might be useful: UC Cooperative Extension

from what I understand Cherimoya can be evergreen OR deciduous. Guess it depends on variety, climate and other conditions.

In any case, I wouldn't worry too much until it warms up.... hey, wait a minute...