Question:
When you took this walk, was your phone receiving a WiFi signal? Perhaps your home WiFi?
If so that would explain the problem.
If connected to a WiFi signal When you took your walk, you would have walked out of range of that signal! Your fitness app would still record your steps, but you would have lost your GPS signal so your miles would not have been recorded!
Suggestion;
The next time you go for a walk make sure your phone is set to your mobile carrier signal, and your GPS location setting is on.
Let me know if this works!