Thanks for posting about this.
I recommend checking to see if you granted Strava consent to process and store your health-related data which includes heart rate data. If you'd like Strava to collect and process health data for future activities you can change your permission settings before your next upload. Unfortunately, our team won't be able to add health-related data back to any activities uploaded while Strava did not have permission to store this data.
Give that a try and let me know if if resolves this issue.