reason for reset ( nrfx_reset_reason_get ) ( hwinfo_get_reset_cause ) not retaining state after boot from bootloader

We are using Zephyr with nRF SDK v2.4.1 on nRF52840. We have a secure bootloader following the example. This all works. Inside the bootloader I can inspect the reason for reset and see it varying with different scenarios as expected. In the main app it is always 0, so has been cleared somehow. I've searched for any 

nrfx_reset_reason_clear() or 
hwinfo_clear_reset_cause
and not found any. Any clues as to what could be clearing it?
If I can't stop the clear, I'll have to store it somewhere.
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