Automatic LTHR detection does not work

I usually get an LTHR update from a 6 min assessment run

I had a harder workout today where I think I was in every heart rate zone long enough. For example, the Stryd sensor detected the new Critical Power, but unfortunately the LTHR autodetection did nothing. Apparently I didn't spend the required 6min in the 5th heart rate zone, but only 5:33 (someone wrote here that it's needed), but I'd guess that's not it. 

Would someone post here how long and in which heart rate zones it was during a successful LTHR autodetection please?

A 6 min assessment that gave me a LTHR update:

Thank you very much, I'll try if 6min in 5th zone is also a LTHR autodetection trigger for me!

I got an LTHR auto detect last week for a 5x600 workout in the middle of a 14 km run. Only spent 4 min in zone 5. 

I discovered that at the end of July it gave me an LTHR update at these values. Since then one guided test and nothing else.

Spent plenty of time in all 5 zones this morning but didn’t get an LTHR. I suspect it was one of two things. 

Either it was detected but it was too close to my previous LTHR for the watch to bother notifying me, or I just didn’t run close enough to my threshold pace. 

(My threshold pace is 4:19/km but this morning I was doing 1000 m repeats at 4:00/km with recoveries around 5:45/km so I was running on either side of my threshold pace but never really *at* threshold.)