Hello
I am sorry for the late answer. Your report is perfect, it allowed me to spot the bug. I reported it here
https://github.com/kkarpieszuk/wc-price-history/issues/80
In cases like yours if you install plugin and start tracking the price for the first time (by changing it to the lower price ans start promo) it stores previous price and new price for the very same day.
Then when plugin tries to find lowest price before this day, there are no entries so it takes the lowest price from the day when promo started (so promo price).
I fixed it partially and I will release the fix soon: it will be working only for new installations and it will on the first history save, save also old price for the day before.
For the already existing sites the fix is not easy but doable, but first, please tell me: how many products does your site has, roughtly?