Lactate Threshold during a Track session is much slower than rep speed

I wouldn't believe a LTHR and FTP that was derived from workouts that included rest. That's not how LT is described in literature. Unless you did a CP30 assessment, I'd reject any figures Garmin comes up with for LTHR.

guessing...are you using a footpod ?

and another guess is that all measurements done are based on GPS data

but 40s is way off

What kind of effort were you hitting on those 400m reps? Is that your expected lactate threshold pace? If you can’t hold 3:25 pace for an hour of racing, then you weren’t training at lactate threshold, and this would explain some of the discrepancy. What times have you run for races?

No footpod, just GPS

effort was 9 out of 10. Definitely anaerobic. Recently I’ve hit 19:17 at 5K and 40:23 at 10K. HM in Feb was 1:28:5x. 4:11 might be right for L/T but my question is more how can it be worked out from that session!

It seems there is some confusion about what LT is. LT is the speed/heartrate/power that you can produce for about 30-60min. Obviously you can run 400m much faster than that speed ;). The best test for LT is to do a 45min all-out constant effort and take the average speed/hr/power. If you show your watch how fast you can go for 82s it will have to extrapolate to what your speed might be for a 45min effort (based on known relations in average runners).

The numbers provided by the watch (4:11min/km) seem reasonable for somebody that does 3:25min/km on 400m with 9/10 effort, but can only be a guess.

It doesn't make sense for that type of workout to produce an LT estimate. It's certainly nothing like the guided test nor the types of runs I've done in the past that auto-generated a result.

When I've gotten LTHR estimates it's been on runs where I was doing something like 2x3 miles at around 10k pace and the result was usually something close to my half marathon pace and HR.

BAD.
HR strap or OHR?

Ahh sorry! Misunderstood the question. Impressive to me that it could estimate it, and likely very close to the real value based on your bests. I’d calculate 4:09 AnT off your race times. Had you done any other faster running with the watch? Any races with it?

These are the fastest reps I've done but I have worn it for a couple of races. Fastest was 5M/8K in 31:29.