For the first test (April 12th), your blood lactate rose above 2mmol/L after 10 minutes, and above 4mmol/L by 15 minutes. Did you warm up before the test, or just jump right into it? Give yourself plenty of time to gradually warm up – at least 20 minutes very easy.
If the pace at 6 miles per hour / 10 minutes per mile is comfortable for you, my guess is that the warmup is the issue with your first test. By 30-40 minutes in, your aerobic system had time to come online, and consumed all of the excess lactate & pyruvate. So, that would suggest that you might still operating below your aerobic threshold at ~175bpm.
The second test, though, seems to have a decent 15-20 minute warmup, although the initial 1.8mmol/L reading at five minutes seems a bit high.
I’d suggest doing another test. Don’t even try to get an anaerobic threshold number. Instead, focus on the lower-intensity aerobic threshold. Start off with a long, very very easy warmup, and gradually increase the intensity. Also, remember that lactate test strips have a shelf life; maybe buy a new batch.