As suggested here, after confirming apps and iOS were fully up-to-date and restarting both iPhone and S6 Apple Watch, initial support staff routed my call to Health App Support folks.
Cardio Recovery was initially named "heart rate recovery", and changed during one of the recent updates. Previously heart rate recovery had no problem calculating this parameter regardless of when you "ended" your work out. According to the health app support specialist this now requires specific manual timing when to END your work out. Stated he would email links to the articles that explains this. He seemed to believe that the information was out there easily obtained. I stated on just one post over 900 people had been having similar problems with that one parameter not functioning correctly as it had in the past, by tapping on "me too".
Here was the answer the health app specialist stated would correct the workout app not collecting data for cardio recovery. When you are finished with your work out (for me a run), and you began a cool down period, you must swipe right and click on END, Immediately. This initiates the collecting heart rate data from the Apple Watch, in my case either from the workout app or the Nike Run Club App, or both, and starting the one and two minute countdown for heart rate recovery calculations. I will test this this evening on my evening run, which by the way may require at least 20 minutes, but Apple specialist was not entirely sure about this. In the past it made no difference what parameters you used for your workout on an "outdoor run".
For whatever reason I was presented with selecting a time for a workout one day and just selected 15 minutes, although I usually run 30 or more minutes, but change it to 22 minutes, anything 20 minutes or more. I wondered by selecting 15 minutes heart rate data for calculating cardio recovery stopped because it was below 20 minutes as a goal? My last entry for cardio recovery was on June 19, 2023. I have data since the first or second Apple Watch and dates back almost 7 years, and before that data was collected from Nike Run Club app on my iPhone with GPS for the past 12 years, which is imported into the iPhone health app as one of the sources.
If you go into the health app on your iPhone, select browse at the bottom right, then select heart then one of the subcategories of interest and scroll to the bottom you'll see all data, and sources for data. You might see a bunch of different sources, but may be coming from the same source, as anytime you change the name of your Apple Watch it will become a different source of data in this location. If you delete any of those sources it will delete the data that was collected under that name given to your Apple Watch. It would behove one when getting a new Apple Watch be very specific to identify what watch it is to avoid confusion here, and for future reference. I named my watch "S6 Watch". Makes it easier to identify with a shorter name, and I only use Apple Watches, so dropped the Apple.
Apple support health app specialist did send me two links discussing how to use the workout app in coordination with the health app on one's iPhone so all the parameters collect data and update correctly.
In the past this was not necessary to know any of this and performed automatically. Why an update would now require one to follow specific directions when to END the workout doesn't seem to be an update. Suspect this will yield more accurate results. To avoid confusion perhaps swiping right and hitting END on the workout app on your Apple Watch avoids confusion when you actually ended your work out and to initiate the start of collecting cardio recover data for one and two minutes.
I pray this information I've been trying to find an answer to since June 20, 2023 with no success anywhere on the Internet or here in the Apple community will be helpful to others.
Best, Seth
Track your cardio fitness levels - Apple Support
Monitor your heart rate with Apple Watch - Apple Support