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Admin of beta tester group has confirmed that there is battery issue in 184.108.40.206. But many people have said that the battery life is same with MIUI 10 or even better than MIUI 10.
You can wait for next update or make a back up and do factory reset or go back to MIUI 10. You can go back to MIUI 10 by unlocking bootloader and using Mi Flash. Refer this thread for tutorial of unlocking bootloader and flash rom. https://c.mi.com/thread-1857937-1-1.html
The battery drain issue is known issue in MIUI 11, mentioned by admin of testing group right after release. Please wait for next update since it might improve battery time. Please note that next update will be Android 10 and devs are working on it but no ETA for now