LTHR auto-detection, maybe some sanity check should be added?

The max HR is set manually and the automatic detection is set to off as it detects automatically numbers that are way too high. Probably this can also affect the LTHR detections to be way too high, but like I've been saying, if you have set your max HR to be X, would be nice to automatically just reject if they detect LTHR that is too close for comfort to max HR. 

If you disabled automatic MaxHR detection I would suggest disabling LTHR auto detection too.
It doesn't make sense to detect one without another.

As I wrote earlier, couple months ago I had situation that my watch started to detect my MaxHR very high (above 200) and LTHR also high (above 170), it took my watch couple weeks to stabilize and return to previous numbers (MaxHR between 170 and 180 and LTHR above 150), and it stays hard on it.

Yes disabled it now after it also started to give numbers that I don't believe.

But that's also interesting thing. I don't remember them ever detecting LOWER number than previous and seems like they just little by little make the numbers too big. Has someone seeing these detecting lower max HR or lower LTHR than they currently had? 

Don't know what would cause this situation, I haven't done anything very special in the training and is already over month from last time I was sick. Hard to say when one could even try to put these back on, so probably just need to keep them off. Maybe a new watch is a time to try them again.

Now setting the values manually is hard as it's been far too long from my last real test to measure the LTHR (or max hr) that should probably think about doing it this year to get newer data to adjust the numbers manually.

You should remember that LTHR will change in time with your condition growth what shouldn't happened with HRMax.
If I would be in your situation I would switch these functions on and observe what will happen in next 2 - 3 weeks.

That doesn't make any sense. Why would you disable max HR automatic detection? If you reach a heart rate higher than 178 then by definition that is your max HR.

Well, if the algorithm would work like that, it would be fine.. But it doesn't.. You go for a long run, HR being nowhere near the max HR.. it detects new MAX HR that is based on some crappy algorithm that it NEVER measured... 

And even if it would measure, it could be measurement error, if there would be some unexplained spike to 210, that should also be detected to be erroneous. 

How Are High Heart Rate and Max Heart Rate Values Calculated on My Garmin Watch? | Garmin Customer Support 

"Auto Detection can calculate your maximum heart rate using performance data recorded by the watch during activities. Any activity with a higher recorded heart rate can increase your max heart rate, but max heart rate can also adjust upward or downward based on a proprietary algorithm designed by Firstbeat Analytics. Your watch can automatically make those adjustments based on any running or cycling activities after you have a VO2 Max Estimate for that activity type."