I am sorry to hear you are having this issue. If this is still occurring, I would recommend trying these first steps:
Update the SW to the latest version 19.34, using Garmin Express.
Ensure your HR is still recording throughout the activity. If this is dropping out, this will effect this metric.
I would recommend trying a hard reset. For this, hold the Light button until the watch shuts off, as opposed to selecting power off. Then power this on again.
If this continues, I would recommend restoring the defaults for the activity app, if it is only effecting Run.
