Indoor run & indoor walk

You may have to set a custom step length, but I am no sure if Fenix 8 has that option.

Another thjng to try is using a foot pod and calibrating that manually (walk a measured distance, count steps).

You may have to set a custom step length, but I am no sure if Fenix 8 has that option.

It will not help. The Custom Stride Length is not used for anything else than the daily distance estimation based on steps. From the documentation at The Step Distance Recorded on My Garmin Watch Is Wrong | Garmin Customer Support: 

NOTE: Setting a custom stride length will only impact your calculated step distance. Your custom stride length does not have any effect on the distance recorded for timed activities.

What can sometimes help, is doing some outdoor walks using GPS, but TBH, I think the algorithm for the indoor walk is simply flawed. It relies on the indoor running / treadmill pace and distance estimation, which is cadence and acceleration based, and the factor for adjusting the pace as a function of the cadence and/or acceleration is largely overrated. It means a small change of the cadence or the acceleration (i.e. stronger/weaker jumps, or stronger/weaker arm motion) changes the pace and distance estimates over-proportionally. Hence at a slow walk, the estimate will be far too low.

Thanks for that information. With my FR955, the indoor walk distance is so far off that I want to try using a foot pod that I calibrate with a step length that is correct for indoor walking (calibrated by walking a known distance). I have no results yet. Haven't had to do lunch walks pacing around in a (large) building since I got the idea. 

Its a shame that they can't add a calibrate feature to the indoor walk that is seperate from the Treadmill calibration?

I have a Garmin. Foot pod for over 7 years on my running shoes that I also use for my treadmill walking. I took it off when I noticed the issue but that didn't solve it.

I walked 20 minutes at a 3.5 mph (5.5 km/h) pace today and manually counted my steps. Here's the graph of it.

During the dips, my steps were all accounted for on my watch. My watch recorded a 1.52 km activity while my (calibrated) treadmill recorded at 1.83 km activity. My cadence was constantly over 130 spm. There is definitely something wrong with this version of the firmware and indoor walking.

So I tried a short treadmill running activity. The speed never dipped, so it's just the indoor walking activity that is problematic.