Isn't this simply how the buy box works?
If you are not the lowest price (with some consideration given to other factors, like reviews and delivery times), then you will not be in the buy box.
If you are not the lowest price (with some consideration given to other factors, like reviews and delivery times), then you will not be in the buy box.
Even if still the lowest price, Prime badge/buy box will be removed if price increases higher than some hidden algorithm allows.