How to create a custom Date Formatter for grouping in Views?
Views uses the rendered value for the field, including the field wrappers to do grouping. In your case, views is treating each date field differently.

There are 3 ways this can be fixed. One of the following methods should do it.


Remove the tags from the rendered output.


Using a custom template and a preprocess function, you can strip all markup and print the output only.


views-view-fields.tpl.php 
template_preprocess_views_view_field

Turn off the RDF module. Views will take care of the grouping itself. The RDF module adds extra metadata to your markup, which makes views separate the grouping as shown in your example above instead of grouping the similar dates together.