For posterity’s sake:
The issue is related to USB configuration.
In firmware revision A05 Quectel added support for UAC to USB configuration (see release notes, 4th item at page 4 is "Supported UAC by AT command AT+QCFG=“usbcfg”).
Supporting UAC made it necessary to add one more field to “usbcfg” parameter.
Until revision A04 configuration “usbcfg” was made of 8 parameters (vid, pid, diag, nmea, at_port, modem, net, adb).
Starting from revision A05 configuration “usbcfg” is made of 9 parameters (vid, pid, diag, nmea, at_port, modem, net, adb, uac).
Switching back and forth between revision A04 and A05, and changing usbcfg params, it becomes clear that the two revisions store such parameters in two different NV memory locations.
Apparently the addition of the ninth parameter forced Quectel to move the configuration to a new memory area, but they didn’t think about keeping the old configuration.
Modems manufactured with revision A04, when upgraded to A05 or later, will set usb configuration to “all usb ports disabled” (due to new memory area not initialized, I suppose).
The issue cannot be replicated with modems manufactured with revision A05 or newer because they would have the memory location of the 9-params configuration already initialized.
After more than a year and hundredths of hours spent by my team to demonstrate the issue to Quectel technicians, a few months ago they were able to reproduce the issue and just said they do not plan to solve it.
In my opinion Quectel have put very little effort in trying to understand the problem.
A software problem, causing modems to become unresponsive, will not be be fixed.