HRV, low state for a long time

last year I never had HRV below baseline. 10 days ago I ran a half marathon at the max (tried to break PR) and since then, it actually dropped below and stayed below. I had two easy 5km runs and tried to rest to see if that makes it better, but still below baseline.

DaKo said:

10 days ago I ran a half marathon at the max (tried to break PR) and since then, it actually dropped below and stayed below

On Garmin Connect, add the nightly averages to the HRV status. This might give you some insights.

For example, in my case, I was training hard across indoor cycling and running and preparing for a 5k race. My HRV reflected the intense training, and the daily average is rebounding after the race (11/19). 

So I am expecting to go back to balanced in the next few days.

Same. Started end of September when I finished intensive training (sort of rest period).

On the same period sleep became really bad. 

Recently started playing hockey (pretty intensive), fixed my sleep, now it is back to norm. Not sure which one worked, but I think generally assessment was correct

Same here and the same happened last year, although in October. But having read a bit, it seems HRV is seasonal. Worth reading analysis from Marco Altini, he is selling a product but there is lots of info on his twitter feed and website. 
The Garmin grey band doesn’t seem to drop straightaway even if hrv falls, if it is seasonal you’d expect garmin to work that out and lower the banding in anticipation. 

There's probably something to the seasonal HRV. I'm not training hard at all right now. The drop in VO2Max suggests that. But with the fall and cool weather, my HRV has been down for over a month and the baseline doesn't care... Take a look for yourself.

Yes similar to mine. I wonder if the change to daylight savings time makes a difference as well. I guess Garmin has all this data and could easily check if it’s the same for others in the northern hemisphere and change the baseline algorithm accordingly.
But then if you naturally have a low HRV at this time then maybe the baseline is correct and you just take it easy for a while. 

Same behavior for me as well. I had a sinus infection for most of October and my HRV reflects that (dropped precipitously at the beginning of the month and was low for the entire month). By the beginning of November I was completely recovered but my HRV hasn't. My sleep hasn't noticeably changed, and my training load has actually lightened (even after I recovered). In a 7 or so week period I've had maybe 6 days where the HRV briefly went into balanced before becoming unbalanced again.

I don't know if my HRV baseline is seasonal. I've had this watch last winter and other than currently, my HRV was lowest in January/February and highest in mid summer, but I don't know how much of it is due to fitness increasing vs. seasonal variations.

Same here (note: I have a Fenix 7 pro, sorry for posting this on the Forerunner 955, but I think they share the same software for HRV).

Training for a marathon.
HRV was in the baseline range (nothing unusual) right until a week to the race. I thought I caught a cold or something because I wasn't feeling that great but the training load was low because I was tapering (work / life stresses were pretty much the same).

Do the marathon, perform poorly and I attributed that to the ``cold'' or whatever I got given the HRV below typical values. 
In the days after the race, HRV goes up again and looks like is getting back to normal, but then it drops again even lower. 
Now the training load is constantly at ``Strained'' level because my HRV is so low, but that's not a reflection of the training load at all. I'm running less than half the mileage, all easy pace, and no changes in external stress factors.

I read something about seasonality, shall I re-calibrate?

for me it is still the same. I had covid19 last week, so the drop was expected. HRV has come back up again after that, but still stays below the baseline.

in October it looked like this: