[ios] fix changes from mapbox#5987

Author: yojique

platform/ios/src/MGLMapView.mm

@@ -2922,6 +2922,8 @@ - (void)addAnnotations:(NS_ARRAY_OF(id <MGLAnnotation>) *)annotations
             MGLAnnotationTag annotationTag = _mbglMap->addAnnotation([multiPoint annotationObjectWithDelegate:self]);
             MGLAnnotationContext context;
             context.annotation = annotation;
+
+            _annotationTagsByAnnotation[annotation] = annotationTag;
             _annotationContextsByAnnotationTag[annotationTag] = context;
 
             [(NSObject *)annotation addObserver:self forKeyPath:@"coordinates" options:0 context:(void *)(NSUInteger)annotationTag];