https://bitcoincharts.com/charts/bitstampUSD#rg60ztgSzm1g10zm2g25zv
Your link says bitstampUSD.
You were complaining about Coinbase, so in the "Symbol" drop down on that website I selected GDAX.
Bitstamp never moved as much as Coinbase did on that date, therefore my GDAX chart looks different than your BitStamp chart (as expected).
I cannot see $19,xxx in the data for the 7th or 8th
I just ran that exact API call (copied and pasted from your post). I DO see $19,xxx in the data
I'll add some spaces in the data to isolate the values so you can see them better:
[1512664200,17900,18300,18200.99,18014.87,129.22107235999997],
[1512664000,18028.41, 19697 , 19100 ,18200.99,740.969024269999],
[1512663800,18182.17, 19697 ,18299.93, 19187.98 ,902.6553972399992],
[1512663600,17639.94,18775,17639.95,18299.92,1077.2144500899974]
In the row identified with time bucket start time of 1512664000
I see high=19697 and open=19100
In the row identified with time bucket start time of 1512663800
I see high=19697 and close=19187