Since the latest Update to WP6 some of the menus are empty because in the table term_taxonomiy (taxonomy = nav_menu) in the column “count” the value is (wrongly) set to 0. Even if there are menu items available.
3 workarounds for solving the problem temporary (24 to 48 hours usually) are:
a) setting the right count in the table
b) manually adding one menu item in the backend and saving the menu (= sets the count right again because it triggers point c )
c) running wp_update_term_count_now() in a script
Still, this does not solve the problem for good or tell us what triggers the count set to 0 again or why it was set to 0 in the first place right after the WP update.
Any suggestions / help / ideas will be appreciated.