NRF51822 16KB or 32KB RAM?
How much RAM is on the NRF51822 devices? There is a 16KB and 32KB variant, and not sure which one is used on your devices.
All MetaWear models based on nRF51 series (everything excluding MetaMotionR and MetaMotionC) are the 16kB variant. MetaWear models based on nRF52 series have 64kB of RAM.