Unable to determine lactate threshold

Is your max heart rate set up correctly? Did you really stay within the zones all the time? For those tests it makes sense to aim for the upper end of the heart rate zones towards the end of a section so that you can quickly transition into the next higher zone and not spend too much time below the target.

I seem to get correct results relatively easily by first properly warming up then running hard enough to feel the lactate build up. I've got it triggered both with 3K and 1K intervals. So the key seems to be just run hard enough to reach the tipping point.

This just using the auto detect.

The watch seems to use the friel zones automatically after the test and correlates well with perceived effort.

I stayed within the zones (maxing out near the end of each one was indeed my strategy). As far as I know my max hr is correct. I haven't actually gone to a clinic to do a stress test to determine its accuracy however.

I haven't adjusted any of my hr information manually, I've figured that was part of the reason for doing the test. Is my understanding incorrect?

To a certain extent yes. However, if you've got HRmax auto detected there could be a problem there. I've experienced enough HR spikes to be very wary of that particular auto setting. Far better to fix that manually. Most folk will use 220-age as a start point because that is most common, but not altogether accurate. Tends to overestimate at the young end and underestimate at the old end.

You don't need to go to a clinic to get a max HR; you can do your own testing. Do a good warm up. Find a hill. Run up as hard as you can, then push a bit harder until you really can't put anymore effort in. You should aim to get to that max effort within 3 to 5 mins otherwise fatigue will be the limiter. Cool down. That will give you a reasonable handle on what your HRmax is. Testing for HRmax is very dependent on your fitness level and training state. If you're tired, you're not going to reach as high as if you've had a few days rest. If you're unfit, you are unlikely to reach as high as when you're fitter. So repeat the test every couple of months.

If you then input a valid HRmax into the mix, you are likely to get more valid and reliable response from the various tests on the device. As an aside, I got a LHR result the other day without doing the test. It popped up as I was doing some interval work on the trails.

AFAIK the whole point of LTHR is to adjust your zones based on that instead of HRmax. And the watch does it automatically using Friel zones. The max may vary by fitness and more difficult to hit once you get fitter but according to Friel zones it should be at least 106% of LTHR. The guides on determining this are a bit more vague but my own guess is that a realistic value would be somewhere closer to 110%.

Correct. But if the Garmin is asking you to run in zones based on HRmax then you need to have a correctly set HRmax. You can avoid all that by setting an initial LHR quite easily without having to concentrate on staying in specific zones:
1. Warm up 15 mins
2. Press lap and run 10 mins at a pace you can maintain for 30 mins.
3. Press lap and continue running at that pace for 20 mins.
4. Cool down 15 mins.

Your LHR is the average HR of that 20 minutes. You can then calculate you LHR training zones.

There's people much more knowledgeable than me on here who can perhaps tell you the exact process Garmin follows to calculate that initial LHR. As I see it, once you've got that first one, any subsequent ones are much 'easier' to calculate.

I use the training plans so as a result, most of my runs are done within prescribed hr zones (i.e. 20 min in z2, 5 min in z4, etc.). Would that stop me from getting an updated LTHR?

Maybe I just need to do some hard running for a week or two (off training plan) and see what happens or do the test again. Neither one sounds too appealing right now.

What are the zones based on? Where did they come from? For the Garmin to calculate a new LHR it needs something 'realistic' to start from. That's why I suggest setting one first from a known datum.

Can you post the GC link to your activity so we can analyse further. Basically the algorithm would be looking for a relationship between pace and HR with obviously increasing pace with increasing HR but only up to a point.

I have seen tests fail say if HR goes up but pace goes down. That could say happen if say you did the low HR bit downhill and downwind and vice versa. Or it could mean you went too hard too early. Or the algorithm just does not always work.

Here's the run. https://connect.garmin.com/modern/activity/1897342514#

The part before and after the track is warm up and cool down. I did the testing part while on the track (so even surface, no downhill/uphill). I did do a little bit of cool down while on the track but I think that part is fairly obvious.