Primary battery dies after 5 months instead of 25
A couple of observations:


Perhaps one of the cells was faulty or with less-than-spec capacity, still it had to deliver the same current as the others and got depleted faster. After the voltage dropped to a lower value, the regulator started asking more current accelerating the drain.
For ultra low power applications, 400 uA is a bit high as a baseline consumption. The Bluetooth module and the microcontroller should have sub-uA or few-uA level sleep mode consumption, maybe you can look into optimization. Once I was diagnosing a low-power wireless node, to find that floating input pins on the microcontroller were causing most of the sleep-mode current drain (about 100 uA); after the fix, it got to 0.1 uA.