I’m having the same issue. It started when they instructed me to follow steps because another function of the Fitbit app is broken. HR averages while using GPS, won’t register. After I reinstalled the app (update) not only did it not fix my original complaint, but now, the app crashes when I attempt to set cues for .5 miles.
I say it’s their update because immediately after my husband’s One updated, it happened to him, too.
Furthermore, I mentioned it to support, both my new issue and my husband’s. I even gave them all of his info to save time (back and forth emails). They completely ignored it and never assigned a case number to him or a new one for me.
So, good luck. I’m currently researching Apple Watches.
Just in case a fix is slow to come, here’s something to hold you over. Look in the category under cues; Time Based Cues. You can select your cue in minutes. It worked for me without the app crashing. It’s not perfect but it’s better than being cued every mile.
Good luck!