I don't believe this is a fwupd bug. If g_network_monitor_can_reach() isn't working with fwupd for your network setup, it's a problem with glib or glib dependencies IMO.
- Bug#1109667: fwupd: Unable to refresh firmware database:... Andy Smith
- Bug#1109667: fwupd: Unable to refresh firmware data... Mario Limonciello
- Bug#1109667: fwupd: Unable to refresh firmware ... Andy Smith
- Bug#1109667: fwupd: Unable to refresh firmw... Mario Limonciello
- Bug#1109667: fwupd: Unable to refresh f... Andy Smith
- Bug#1109667: g_network_monitor_can_reach() returns ... Andy Smith
- Bug#1109667: g_network_monitor_can_reach() retu... Simon McVittie