fstorage callback not called

I am trying to use fstorage with the sd backend (SDK 14.0.0). My issue is that, after successfully calling nrf_fstorage_write, the callback I registered using

just never called.

This leaves me with two questions:

1. What reasons might there be for my fstorage callback to not get called?

2. Can multiple modules register SD observers using NRF_SDH_BLE_OBSERVER with the same priority, or does every observer need to have its own priority?

Thanks!