Fitness app not showing activity data, but Health app does

I have an iPhone 13mini running iOS 18.3.2. The Health app correctly tracks my steps, but the Fitness app always shows zero. I have tried the following troubleshooting steps:

• Checked that Fitness Tracking is enabled (Settings > Privacy & Security > Motion & Fitness).
• Tried to verify Fitness app permissions in Settings > Health > Data Access & Devices, but inside Fitnessit appears: No data found.
• Uninstalled and reinstalled the Fitness app.
• Restarted my iPhone and updated to the latest iOS version.

Despite all these steps, the Fitness app is still not displaying activity data. How can I fix this?

(I do not own an Apple Watch)