There are three weeklys, two with two releases and a stable version. All in all - 4 to 6 potential ROMs of/with MIUI11.
You think you provided enough information by saying MIUI11 and MI6? Really?
Fast battery discarge has been beaten to death on this forum. Could be as simple as a clean install and gallery and music app caching files, to a silly decision to block a service/app making other to work overtime, to an incorrect install, to a very improbable real bug.
Which of these do you think the devs have to read your mind and/or guess?
