Significant suggested pace changes after 21.19 update

Yes, I have the same issue with version 21.19 on my Forerunner 955. After the update LTH and MaxHR are completely unusable and DSW and Garmin Coach are a way off. I use a HRM Pro+.

I had the same on my Forerunner 955, my suggested pace for base run went from 6:05 to 5:20 with Threshold going to 4:15 and VO2 max to 4:05. For the base one i am pretty happy about the adjustment as i found the 6:05 pace a bit slow but i have to admit is not always an easy pace but the faster paces are somehow impossible to keep up for me. As i am currently training for a marathon i am kind of afraid of overtraining with this sudden increase.

Now that you mention it, yes, sort of. About a month ago, my DSW base runs were suddenly 6:15 /km rather than 6:35 or so. That's not such a big shift, but it is much harder to keep up for 50 minutes or whatever. I was a bit surprised but, initially, I thought it was an acknowledgement that I had made some progress. However, this step change did probably coincide with the update to 21.19 and I notice that autodetection of max HR has been turned back on.

There are other threads on this topic. Many users have seen unusually high LTHR and MaxHR after the 21.19 upgrade. I think Garmin is investigating the issue so hopefully it gets resolved soon.

I am hoping that Garmin is investigating. In this thread Garmin-Sierra asked for people who have the same problems or "believe" that their LTHR measurements are inaccurate. It would be nice to know if Garmin is working on that since I will not update until this is resolved.

One more interesting note is that they mention LTHR is measured now every time even without chest strap. But in my case I'm always using a chest strap, and I'd expect that Garmin should rely on the chest strap if it's used, but I still got those changes in paces.

I know there was a thread where I thought that the watch is always used for the LTHR, I think Garmin-Sierra then answered that if a chest strap is used it should rely on that.
Sadly I can not find that thread right now.
But since the algorithm that decides if the strap or the watch is taken for heart rate seemed to be very buggy (selected the watch most of the time in my case) I am afraid that this happens also for the LTHR. For the heart rate after I experienced the problems, I configured the watch to always use the belt, but that does not necessarily mean that this applies to LTHR as well.

Garmin are investigating the inaccurate LTHR autodection. For example, they have asked me to participate in collecting extra logging data and mark instances where suspicuous LTHR/MaxHR detection occurs, which I've done. So hopefully they are able to do something after getting enough log data and analysing the logs.

It went the other way with me. My threshold pace used to be ~ 7:30. Now it's saying I should be running 8:40, which is both perceptively and by heart rate way too slow for that kind of workout.