Versa 4 not recording zone minutes for brisk walk unlike Charge 5

[alok18]

I have been mostly using Fitbit Charge 4 or 5 smart bands for the last 7 to 8 years to monitor my health parameters / fitness goals. 
While doing my walks, the Fitbit bands used to record brisk walks (zone minutes) for about 75% of the time. I walk at 100 steps per minute  or above (and can't talk and walk simultaneously at that pace) which, I presume means a brisk walk. That enabled me to meet my target of 22 zone minutes per day.
I purchased a Fitbit Versa 4 recently about one month back since the battery of my Charge 5 had to be charged daily only after 15 months of use. However, in the Fitbit App now I am unable to log any zone minutes for the entire 30 minutes walk, though I am walking at the same pace. Sometimes I get one or two zone minutes. Other health parameters like Sleep score, Resting Heart Rate. Spo2 and Breathing Rate are more or less the same as in the Fitbit Charge 5 earlier.
What could be the reasons? Why there is this difference in zone minutes between Charge 5 and Versa 4? I am unable to sync my Charge 5 to the Fitbit App now since dual sync is not possible. 

[LZeeW]

Hi @alok18.  The Versa 4 has a different heart rate algorithm compared to your Charge 4 and 5.  Fitbit put this same algorithm on the Charge 6.  I suspect you would have the same experience if you upgraded to a Charge 6 instead of a Versa 4.

[alok18]

Thank you, LZeeW. However, does it mean that I am not getting my targeted zone minutes, and that is not good for health? What is 'more accurate / correct'? Charge 5 or Versa 4? 

[LZeeW]

@alok18   Your description of your walking pace actually meets the definition of a brisk walk.  The only way to test the heart rate accuracy of a Fitbit is to compare it with a chest strap heart rate monitor.  A chest strap uses your ECG, not an optical heart rate monitor.