Low power mode drains my battery even faster

Specs:

iPhone 6

iOS 11.3

16GB

I'm not sure which topic to post this under but whenever I turn on low power mode on my iPhone, the battery drains a lot faster than with it off which is incredibly weird and annoying.

I don't know why this is happening and sometimes even at 10% or 12% the battery drains completely flat even then in low power mode.

Is there any suggestions why this is happening? I checked with Apple and they said my battery is still in good condition with next to no wear, my device is also years out of warranty and this feature is no longer useful for me anymore. (All it does now is slow my performance down which is expected but drain my battery faster along with it)

Thanks.