"Have you tried updating your Apple Watch to the current watchOS version?"
Yes. Most mornings I do a slow run with my dog and a longer/faster run alone. The heart rate chart did not work for the first run but it did for the second. Strange!
"I would try unpairing the watch, resetting it and pairing it back again."
I might try that eventually. I'm not sure having the chart is worth that effort.
"I'm wondering if this could be a bug after this week's update."
I don't think so. It was flaky before the update and it's flaky now.
" I was missing the weather at the bottom of the workout for weeks, I think after 15.1.1 was released, and that was fixed with 15.2 and watchOS 8.3."
LOL, I didn't even realize it was missing. I looked and it seems to be there sometimes and absent other times.
"I'd suggest cleaning the heart rate sensor, and trying first a restart of both phone and watch to see if that makes it more reliable."
I've done restarts. I haven't tried cleaning more than a rinse. I'll give it a shot.
Thanks all. I'll let you know if anything changes.