I think you're fine... assumption for age (under high 40's?) ... and given the ride is mostly flat, light rolling - you're not really spiking into long segments at/near AT (estimated...)
if you look at the median of the group B segment, it would seem to be in the mid-160s - a good hard, steady effort...
You're obviouslly not getting shelled in the B group ride - so continue on... and build into more assertive riding role when you feel it right.
The early hour at lower HR, gives the body plenty of time to build into an effort.
There lacks any 'cool-down' data, so I'm hoping that you have 20 min of cool-down after the Group B effort. DOn;t just jump off the bike and get in the coffee line.
That cool-down is critical in allowing the best possible recovery from the entire ride. 'Cool-Down" !!!!
zone 2 - given the observed HR - mid-120 backed down to teens, over 15-20 min... lower is even better for the last 5 min of cool-down.
check your heart rate and BP, that evening before bed, and again on waking... if it's spiked at either check, 'recovery ride' the next day - no higher than 120 ish 45min- 1 hr
Ride On
Yuri