Is there any noticeable improvement from 11.12 to 12.27 ?

I feel you.

When you ran with F6, did you compare the cadence, stride length, pace, etc ?

If Garmin thinks it's not a SW issue, then it must be a HW issue.

When you ran with F6, did you compare the cadence, stride length, pace, etc ?Go and request for a replacement watch. Ask what's the firmware of the replacement watch.

If given a choice, I would rather stick at 11.12. I accidentally upgraded to 12.23 when I connected watch to laptop for retrieval of watch files to submit to Garmin 

I feel 12.23 is worst off than my 11.12.

Hi, I saw this thread and thought it may be relevant and it was interesting as it was the other way around to you. See the end comment and maybe that is applicable to you?

https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-955-series/311711/fw-12-27-fr-955-way-off-in-measuring-total-distance-pace

Thanks for the info. Since the distance on my FR955 was spot on in multiple activities in July, I haven't changed anything on the watch initially. After it started acting up in late Sept, I carefully changed back and forth few parameters (such as smart vs 1s, GPS mode, activity type, etc.) but it made no difference. BTW, I did upload my FR955 GPS track from Oct 12 to Strava and again the distance on Strava was 6.28km compared with the 6.26 on my Fenix 7s (that I wore on my right wrist) and the 5.83 km of the FR955. Garmin is looking now into my issue and hopefully will find what's wrong. I can't imagine a HW defect that yields good tracks but wrong distance, but I don't see many complaints of issues like mine which means it is may not be a firmware issue.

Just check the settings on this 3D thing as sometimes the FW updates change things we don't expect as defaults... not sure if it can or may have any effect but anyway...for example the temperature recording suddenly went to off on one of the FW updates.

Puzzling 

My 3D distance and speed are off.

Ok , so that can't be it...one (more) thing eliminated..

@Idgsd1. I have a similar issue however,  it is opposite to yours. My watch consistently measures the distance & pace on the higher side (it shows more distance covered then it really is). Please let us know on this thread about the reply by the Garmin team on the current issue. Thanks. 

Will definitely let you know. Can you upload the activity gpx track to your computer and check the reported distance by Strava or one of several other programs on the web. How is the reported distance compared to your watch activity distance?

With the update to 12.27 my race prediction is way off.. It suddenly decreased by race time my huge margin.. 10K at 46 min, half Marathon in 1:46 min.. before it was 10 min more in both races and was mostly accurate 

Which version of the software are they using, I believe mine updated automatically and I didn't even notice it overnight.