In your views sort settings, make sure your Date sort is the first one, because it seems that there is another sort by entity title applied first

#UPDATE After checking the view export:

/* Sort criterion: Content: Post date */
$handler->display->display_options['sorts']['created']['id'] = 'created';
$handler->display->display_options['sorts']['created']['table'] = 'node';
$handler->display->display_options['sorts']['created']['field'] = 'created';
$handler->display->display_options['sorts']['created']['granularity'] = 'month';

You are sorting the results using "Post Date" of the node, not the date field, you should remove this and add the date field instead

In your views sort settings, make sure your Date sort is the first one, because it seems that there is another sort by entity title applied first

UPDATE

After checking the view export:

/* Sort criterion: Content: Post date */
$handler->display->display_options['sorts']['created']['id'] = 'created';
$handler->display->display_options['sorts']['created']['table'] = 'node';
$handler->display->display_options['sorts']['created']['field'] = 'created';
$handler->display->display_options['sorts']['created']['granularity'] = 'month';

You are sorting the results using "Post Date" of the node, not the date field, you should remove this and add the date field instead

In your views sort settings, make sure your Date sort is the first one, because it seems that there is another sort by entity title applied first

In your views sort settings, make sure your Date sort is the first one, because it seems that there is another sort by entity title applied first

#UPDATE After checking the view export:

/* Sort criterion: Content: Post date */
$handler->display->display_options['sorts']['created']['id'] = 'created';
$handler->display->display_options['sorts']['created']['table'] = 'node';
$handler->display->display_options['sorts']['created']['field'] = 'created';
$handler->display->display_options['sorts']['created']['granularity'] = 'month';

You are sorting the results using "Post Date" of the node, not the date field, you should remove this and add the date field instead