This one's new with 8.0.0; I've not seen it before in any of the other firmware revisions or betas....
I have now gotten two separate lactate threshold detections that both had utterly ridiculous heart rates (exactly 80 in BOTH cases) after a fairly hard run. Both were potentially long enough and in the bracket where they could trigger detection, but there is no possible way the threshold heart rate detected was 80; that would have occurred within a minute or two of starting the workout, and of course I didn't return to an <80 heart rate until long after it was finished (and after the popup at the end of the activity.) They came back-to-back on successive workouts as well.
Something appears to have been severely broken in terms of these metrics in the latest firmware release.