Cardio Fitness Score drops drastically on Ionic

[Hashknife]

I am 71 and have been using the Fitbit Ionic for 2 years.  My Cardio Fitness score is currently in a range displayed from 42 to 47; my resting heart rate ranges from 57 to 60.  These numbers have been consistent for many months.  My exercise consists of walking and weight work/ training, and a very occasional run (jog).  My walks and jogging are in very hilly terrain.

Last week, I complete a 5 k run (jog) on less hilly terrain, and when I checked my cardio fitness score, it had dropped to 34.  I was astounded, amazed, and dismayed since before I completed that jog, my score was 46. 

I've read the community posts regarding drops in cardio fitness scores, and still would like some clarification regarding this phenomenon on my Ionic.  

My Cardio Fitness Score has always been displayed in a range, and now it just says 34, no range.

My resting heart rate, weight, and other factors have not changed.

I updated the firmware last week to 27.7. 2.1.9 which may be the issue.

Please let me know your thoughts.  THANK YOU!!!

Moderator edit: subject for clarity 

[JohnnyRow]

When it shows a range, that is an estimate.

You get an exact number after you have done a GPS-connected run.

From How do I get a more precise estimate of my score?  

"If your Fitbit device connects to GPS, we can provide a more precise estimate of your score using the relationship between pace and heart rate during your runs. This is because individuals with higher VO2 Max have a lower heart rate while running at the same pace compared to individuals with lower VO2 Max. If you're able to run at a comfortable pace for at least 10 minutes, track a run with GPS. We recommend running on flat terrain as much as possible since only the flat sections of your run count toward your score estimate. You may need to go on several runs that are at least 10 minutes in length to affect your score. "

Note it might take several runs.  I expect your previous jog was not enough to change score, but the 2nd one was.

And it will be more accurate if you ruin on flat terrain at least 10 minutes.

[Hashknife]

Thank you for your response.

When I have completed my previous walks and runs, I'm always using hitting the GPS, and this last run when the score dropped is no different.  I'm confused why the score changed and nothing else, including using the GPS, has changed.