Masterpress 1.3.6 is a compatibility release. It resolves an edge case with shared fieldsets not working for MySQL 8 when the fieldset is limited to more than 1 post type or taxonomy or excluding any post types or taxonomies.
WOOF_SimpleCollection :: flatten( String $separator = ", " )
Overrides WOOF_Collection::flatten to be appropriate for a simple collection (where literal types are stored, not objects with properties). This method flattens the list of items into a string separated by the given separator.