Fastboot command stucks and goes to disk sleep
I finally discovered the source of the problem - another process had apparently claimed the USB device. In my case, fwupd was automatically connecting to the device, attempted some kind of USB transfer and failed. However, it didn't release the device, which is why fastboot was stuck and waiting for it to be released.
Stopping the fwupd daemon, fixed the problem.
I've opened a bug report with fwupd here ( https://github.com/fwupd/fwupd/issues/3187 ). Meanwhile, if you want to be able to use fastboot, make sure no other process is using the device.