15.20 and 15.71 - Heart Rate goes bonkers and can't be trusted

I've been able to really nail down a repeatable issue that should be easy for Garmin to replicate and fix.

Here's my routine:

1. Walk about 15 minutes to the place I work out outdoors. I use the Walk app to track.
2. When I get there, I save the Walk, and then Start the Cardio app.
3. When finished, I walk home and keep the Cardio app running until home.

What happens is that whenever I start walking home, my heart rate goes up in the 170s. Normally, this is low 90s unless I've just finished a workout/run. Then it might be in the 110-120 range for a while depending on my walk pace.

The 15 minute walk there records fine.

See the screenshots below.

Watch is on snug.