Heart rate derived metrics not working

I did a ride without power today. I have heart rate derived metrics enabled, however it’s not analysed my ride, so I have no xss or anything else for it. I’ve tried to figure out if I’ve missed something - a setting somewhere maybe, but I don’t think I have. Any ideas what has happened?

Is the activity flagged due to a gap in the data?

Same happened to me on Sunday, had to guess the xss and input manually onto the planner.
Otherwise training advisor would be telling to train/falling behind. Despite having just done a tough ride.

I haven’t flagged it, no.

Was it flagged automatically? (gap or spike detected in data)
Look under activity details or the activity table for highlighted flag.
If not flagged, post screenshot of the chart with HR included.