The kernel I package with this is another kernel I made just for MIUI. I would guess you are getting less battery life due to the battery getting older or it is just all the cool stuff MIUI adds every week. Just an FYI, these batteries dont have much a life as a whole. With all the development I do, my phone is plugged into the USB almost all the time so I have completely worn out the cells in the upper 10-15% of the battery. When on battery, I loose about 10% in about the first half hour or so then goes back to normal. Take it for what it is worth.
I hate to say it, but battery life did get noticeably worse between 5.18 and 6.1
I never got to try 6.8 but I did ship my phone with it
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