_menu_router_merge_route() in menu.inc converts symfony style routes into Drupal style menu items. Symfony style paths mark placeholders with curly brackets, and the menu_router table stores paths with percent sign wildcards.
Since the menu_router table is still being used to create breadcrumbs and menu tree stuff, it isn't working correctly for wildcard paths defined in routing YAML files. (see #1944910: Convert the 2 menu callbacks in ban.module to a new style controller (and form controller)).
This also causes problems with the a 'title callback' function defined in hook_menu if it takes an argument from the url (see #1938386: Convert contact_category_* pages to a new-style Controller).
Proposed Resolution
Two options here:
1) Change how symfony paths are converted into menu_router entries so that curly brackets are translated into percent sign placeholders.
2) Change the menu system to understand and interpret curly brackets.