SUMMARY
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***


STEPS TO REPRODUCE
1. open system settings, select the "shortcuts" section, then the "kwin" section.
2. select any action, e.g. "Switch one desktop down"
3. Click on "Add custom shortcut"
4. type keys Meta+e. A pop up is displayed saying "Shorcut Meta+E si already assigned to action 'Dolphin' of Dolphin. Do you want to reassign it?"
5. Click on Yes
6. In the lower right corner of system settings, click on "Apply".

OBSERVED RESULT
typing Meta+e does nothing, not even opening dolphin.

EXPECTED RESULT
When I type Meta+e, plasma switches one desktop down.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: openSUSE Tumbleweed 6.3.7-1-default/5.27.6
(available in About System)
KDE Plasma Version: 5.27.6
KDE Frameworks Version: 5.107.0
Qt Version: 5.15.10

ADDITIONAL INFORMATION
 - If between step 5 and 6 I uncheck the Meta+Ctrl+Down default setting, and check it back, then everything works as I expect.
 - If I click on another section of the settings panel (e.g. "About this system") and come back to Shortcuts -> KWin, the Meta+E setting I put on "switch one desktop down" has disappeared.
 - If after step 6 I select the Dolphin section in the shortcuts, click on "Add custom shortcut", type Meta+e, approve the reassign, click on Apply, and type Meta+E, then Dolphin does show up.
 - I observed the same behavior both on my personal Fedora machine and on my work tumbleweed machine.
 - If I assign another shorcut that does not conflict with another shorcut, then it works as expected.