Erratic Stamina readings

Must be related to the bug on training effect.

Garmin thinks you are over workout.

Similar anomalies here too.  A normal average run for me today at a very slow pace and Garmin had me at single digits by the end.  

I dont know if this forum is monitored necessarily, but it'd be helpful to get in touch with customer support and escalate this problem. I did, a couple weeks ago, and it was escalated to the engineers. I shared all my data with them so hopefully it helps them fix it, but I think more people should be doing this as well. I loved my 955 so much before the 12.23 update, and now it feels almost useless.

2 possibilities that I can think of.

1) Your max HR is too low ?

2) Your running HR is too high ? HR data correct ? Cadence lock ?

It's a software problem. It kicked in for everyone on update 12.23. But it definitely affects more than just stamina, and may stem from an HR problem. When the problem started the watch also started lowering my max heart rate every run (auto detect feature) until it was comically and nonsensically low.

I'm on 12.23 now. I don't have problem with training effect and stamina.

I mentioned a lot of times. Your max HR is crucial. 

When Garmin detects garbage max HR, you shouldn't accept it.

I switch off the auto detection and stick to my original max HR. All is fine for me when it comes to performance.

Tip: Do not use the 220 - age formula.

I've tried a few formulas, spanning a range of HRs, as well as the auto max detect, each at least for multiple days at a time. My stamina graph looks like the one OP posted. I did the slowest 3 miles (very short run) I've ever done the other day and the watch called it a threshold run. My watch doesn't offer me workouts anymore because it wrongfully thinks I'm burning myself out. It's definitely a software issue. Everything worked perfectly until 12.23.

I do appreciate the help and tips though.

Edit: I am curious why it wouldn't be happening on your watch. You have the 955 and you use it for running? Do you have any additional apps installed? The only extra thing I've added is spotify, so I can listen to music while I run (I guess that could suddenly be conflicting somehow). And I don't sync the "garmin ski map". Those are really the only 2 personalisations I've done to my watch.

I am not sure what range of HR you are referring to.

What is your actual max HR ?

What max HR did you configure on the watch ?

I attach my recent 25 km run at 75% of max HR on 12.23.

DPSK has a good point here; check your HR Zones to make sure they are correct; I didn't have any issues with stamina on the 12.23' In 12.25 the only thing wrong with stamina was the arrow pointing the wrong way ( it was increasing all the time)and no issues with the 12.27 ( 12.25 and 12.27 are Beta).

I have done some tests for Garmin where I have change my HR to default and that also changed the HR Zones; my stamina in some hard runs looks close to yours due to the incorrect HR Zones.

You can try changing the HR Zone to LTHR to see if it make a difference, or you can take the plunge and register for the Beta and install the 12.27 and see if that corrects the problem.