Turns on instantly after powered, drains batteries, doesn't turn off

Immediately after inserting batteries or plugging my Zoom H6 into a 5V micro-USB power source, the device displays a white screen then turns on (without me using the power-on button). I can use the device, but when using batteries they drained incredibly quickly. The machine doesn't always appear to fully power off, since after holding the power switch and it shutting down sometimes a lit black screen appears. The batteries seem to drain when this happens indicating the machine is still using the batteries. Any suggestions for a fix?