SUMMARY
With 'Screen edges' -> 'Switch desktop on edge' set to 'always enabled' this starts working perfectly fine, for many days mostly, but it always eventually stops working.
This bug has been with KDE for many years (at least 10).

STEPS TO REPRODUCE
1. 'Screen edges' -> 'Switch desktop on edge' set to 'always enabled'
2. Just use the computer for many days, it triggers within something like 180 days (this run lasted 49 days).
3. Hitting an edge with the mouse no longer switches to another desktop. Everything else still works except for 'present windows' in the upper left corner.

OBSERVED RESULT
Hitting an edge with the mouse no longer switches to another desktop.

EXPECTED RESULT
Hitting an edge with the mouse switches to another desktop.

SOFTWARE/OS VERSIONS
Operating system: Ubuntu 20.04
KDE Plasma Version: 5.18.8
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

ADDITIONAL INFORMATION
When this bug triggers the edges and corners still show blue-ish when the mouse gets near it (screen edge highlight).
Desktop switch via the scroll wheel on the background still works.
Desktop switch via the desktop panel in the task bar still works.
I always use 3 rows of 3 desktops each and no desktop switching animations are configured.