LibraryModels.nullMarkedClasses() may not be checked for a class depending on when the class is first seen. Example:

class Bug {
    void use(Object o) {}
    void f(Object o) {
        // First cause dataflow to be computed
        use(o);
        // then use a yet-unseen class marked by LibraryModels
        use(toNull.apply(1)); // False negative - there should be a warning here but isn't
    }
    Function<Integer, @Nullable String> toNull = i -> null;
}

#903 added the ability for LibraryModels to make classes effectively @NullMarked, but this is not being used consistently.
CodeAnnotationInfo.get may be called with or without a Handler, and in either case it caches the result. In this example, while visiting the first use call, CodeAnnotationInfo.get is called with handler == null and so the java.util.function.Function class symbol is cached as unannotated.

Workaround: include the modeled class/package names in the NullAway:AnnotatedPackages option. In this example,
-XepOpt:NullAway:AnnotatedPackages=java.util.function.Function
would make the warning appear properly.