So many times firmware updates from Garmin is like a game of whack-a-mole; one problem gets solved, another pops up. The latest dual updates from Garmin for the Forerunner 235 (WHR 2.6 updates the code for the Elevate heart rate module, 4.2 is a firmware update for the watch os) are true to form.
Improvement: the optical HR monitor is doing a better job during exercise. It seems like the heart rate is being updated on more like a one second than a 10 second or more basis. It locks onto real heart rate faster, is less confused from noise from your cadence (so-called "cadence lock" - where the optical hr monitor locks onto your footstrike cadence instead of your heart rate - why you may be seeing hr readings of 170 or more on easy runs), and is steadier over the course of a run - less spikes, more responsive to changes in effort. The run today was great - just a little spike around minute 45, but it actually may have been real given the increase in pace around that time.
Unfortunately, the bad comes with the good. Prior to this update, my resting heart rate is usually in a narrow band - usually 48-53. I've seen a reading of 40 once, but anything lower than that, or higher than 55 is abnormal.
This morning on waking, it looked as if the module did little or no reading during the night. It returned a resting reading of 50 as I was lying in bed, which is on target. But after the run, it showed a 36 resting heart rate. Not on target.
There have been a number of similar observations on the Garmin 235 forum. Hopefully they can get this resting HR issue straightened out quickly without undoing the good work as far as responsiveness during activities.
Edit: resting HR of 48 on day 2, so perhaps the device is making adjustments based on day-over-day data.
I am running the latest version of Garmin Connect Mobile (126.96.36.199) on a Nexus 5X.