With the battery at that level is normal for that to happen at least sometimes. For me it always happened even on stock MIUI, when the battery goes below 35%, the level starts to drop faster and faster. This kernel has a slight battery drain of at least 1% every hour even with everything turned off and the phone just sitting idle on deep sleep, but it's not abnormal.
I use it along CM13 as my daily driver, used Amplify to turn off some wakelocks, Franco's Naptime to turn on Agressive Doze and Greenify to hibernate a few apps, and the result is an energy consumption of about 60 to 80% in a full day with data and bluetooth always on (connected to a MiBand), a few calls, many messages, some gaming, some browsing and a few hours of music playing.
FS + CM13 = best combo so far.
Enviado do meu MI 3W através de Tapatalk