There seems to be an issue with the is_store_page() function such that it returns true if the post has been saved with 'none' in the Store Page Settings metabox dropdown. Digging into it it seems that the issue is that the "none" option in the dropdown actually stores the value "none" instead of '' in the database.
I did a quick search and found the following, there is a check as follows in class-mp-public.php on line 158:
get_post_meta( get_the_ID(), '_mp_store_page', true ) !== ''
Since _mp_store_page stores 'none' in the database this check doesn't work. For me I modified that check to the following but it would likely be better to simply delete the post_meta instead of saving 'none'.:
!in_array( get_post_meta( get_the_ID(), '_mp_store_page', true ), array('','none') )