I had a device behave like this and it was a firmware bug. The MCU and peripherals did not properly go to sleep- under certain shutdown sequences.
You might try a few things and see what makes it shutdown properly, if that is possible or if Bluetooth activity is part of it staying at a higher drain etc. Or it might draw higher current just for a few minutes after shutdown.
Sometimes a programmer is a bit sloppy with the 'go to sleep' path and forgets to turn something off.
In my case, the manufacturer issued a recall because it caused batteries to go dead and then leak. I had to laugh, the replacement product changed over to a hard on/off switch
instead of fixing the firmware bug.