It was working fine when I installed the build from a hard reset. Then stopped working. Oh well, that's preview build's quality.I didn't think battery saver worked in the TP yet?
Tried soft-reset, OP?
The real, not "slide down to power off" option which is not the same as reset.
You perform the soft reset by having your phone turned on. Then press and hold the volume down + power buttons until your phone vibrates and restarts. Note that you should not touch the device screen during the process (not even when the 'slide down to power off' prompt appears).