SUMMARY
There is a firmware update for my Dell XPS-13-9360, but Discover doesn't show anything.

If I use the fwupdmngr command I have the following output:
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nicola@nicola-XPS-13-9360 ~> sudo fwupdmgr get-updates
XPS 13 9360 System Firmware has firmware updates:
GUID:                    5ffdbc0d-f340-441c-a803-8439c8c0ae10
ID:                      com.dell.uefi5ffdbc0d.firmware
Update Version:          0.2.9.0
Update Name:             XPS 13 9360 System Update
Update Summary:          Firmware for the Dell XPS 13 9360
Update Remote ID:        lvfs
Update Checksum:         SHA1(2209fed7cc24bfbf6e40f2e93e8bb1befa19d641)
Update Location:         https://fwupd.org/downloads/afddc660b9a0f8045feaa4703f8258a5d197dad1-Signed_1152921504627781711.cab
Update Description:      Fixes: - Update Intel ME Firmware to address security advisories INTEL-SA-00125 (CVE-2018-3655), Intel-SA-00131 (CVE-2018-3643 CVE-2018-3644) & INTEL-SA-00141 (CVE-2018-3657 CVE-2018-3658 CVE-2018-3616) - Fixed a potential system hang issue when an incorrectly formatted password is entered at the BIOS Security Manager (BIOS pre-boot password) prompt.
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This is the proof that there is a pending firmware update.


STEPS TO REPRODUCE
1. Open Discover and check for the updates
2. 
3. 

OBSERVED RESULT
Discover response is "No updates available"


EXPECTED RESULT
Discover should list the available firmware update


SOFTWARE VERSIONS
(available in About System)
KDE Plasma Version: 5.14.0
KDE Frameworks Version: 5.51.0
Qt Version: 5.11.1

ADDITIONAL INFORMATION
In Discover is checked the Firmware Updates "lvfs - Linux Vendor Firmware Service"