The issue has not occurred since removing the loose piece of hardware on the modem.
However, I have a second issue that still occurs which has a different symptom: sometimes even after removing the loose metal square cover, there will be times that the mobile data icon is visible on the top bar on the Librem 5 (indicating that the modem is on and is accessible to the Librem 5) but the internet data will not actually be working. In these cases, toggling the modem HKS will reload the connection and get mobile data again.
But for me that is a very different symptom than the modem icon disappearing entirely off of the top bar – basically losing connection to the hardware – and having it not come back even after toggling the physical switch. The “not come back even after toggling the physical switch” part was really stressing me out because it means that I could be unable to access data no matter what, seemingly at random. And that stress is gone after removing the physical metal square.
I daily drive the device so this is very real that after removing that loose piece of metal from the modem, the “very bad” symptom is gone and things are basically as they were before – albeit with the sometimes loss of data issue. But I figure that issue might be the carrier for all I know.