I'm running an nrf52832, using SDK 11.0 and the S332 soft device. I've had instances where it seemed my flash data, stored with FDS, was corrupt after a firmware upgrade. Note that generally this is not the case, but infrequently I have had issues.
I've not investigated the root cause yet. I'd like to confirm that the expectation is that FDS data is safe from a firmware upgrade. I.E. all FDS metadata is stored somewhere where it is not changed by a FW update. Note that I am in the process of upgrading to SDK V12.2.0, so please be clear if this answer depends on the SDK version.
I could not find any indication about persistence of data after FW update in the FDS documentation.
Thanks....