Mamba H743, HGLRC M80 PRO GPS. 

Booted up a 10” project I’ve been working on for several months off and on. Noted the Mag and Baro were not visible on the Betaflight screen. Both had worked in the past. 

Noodled a bit, opened CLI, did a diff and dump, both looked ok to my very un-trained eye. Typed exit and reconnected. Poof! Mag and Baro were back!

I’d just like to know why, please?

Once I’ve got it flying in Betaflight my plan is to shift over to iNav for some autonomous flying, position hold and landing fun. I’d prefer the Mag and Baro operate flawlessly. 

Thanks.

Unless there is a bug in the version of Betaflight you have installed on that FC or there is something on the FC that caused an intermittent short circuit on the FC then it's impossible to know the root cause.

I would check both sides of the board over thoroughly with a magnifying glass and make sure you have the latest revision of Betaflight installed for the version you are running (i.e. if you are running 4.2.x then install the latest 4.2.11 revision, and if you are running 4.3.x then make sure you have the latest 4.3.1 revision installed). Don't forget to save a "diff all" in the CLI before (re)flashing firmware, and make sure you switch on the "Full chip erase" option.