I am not sure. When I change to love route for her it changes to "Ask Miko our for a date", but on harem route and even on love route with other girls it still says ask her out for tonight
Bug is not showing for me. Let's try this for now. Choose Miko love route. Ask her out for a date. Then at night, take her out for the date. Let's see if this clear things up. If this doesn't work, let me know how many love events you have completed with her. You can see that in her gallery