I was yesterday in my local Apple Store, Oberhausen, Germany (One of the biggest, top 3 in germany). There were three people next to me at the Genius Bar with the same problem. Here the process from my understanding:
Genius explains the complete process (backup!, because of erase).
Plugs powercable and an ethernet cable into the macbook.
Runs standard disgnostic on his 9.7“ iPad, everything was fine for me (all checkboxes green).
Restarts macbook and selects bootup from specific server location.
You have to wait 10-20min. After that the macbook gets into somekind of recovery mode and offers a firmware update for the ssd with complete erase of the ssd (backup!). Wait another 10-20min, till firmware update is finished. Everything worked fine, here is the point were faulty ssds fail(?) and you have to get the macbook checked in for repair/ssd exchange (genius said max. 14 days return time, but all customers with me only had to run the update and they were good to go).
The last steps were (genius said „precautionary“) reinstall of fresh macos (you are able to select from your own time machine backup, so my tip bring it with you in the store), genius ran another standard diagnostic, everything fine again.
Done. I was around 1-2h in the store with everything.
Made screenshots before and after, you can clearly see the firmware update.