SUMMARY
With several virtual workspaces configured and the mouse wheel vertical scroll configured to allow switching between workspaces, it does not work if "folder view" is selected. Switching does work in "Desktop" mode. Switching via middle click does work as well. The same issue is described on this forum post: https://forum.endeavouros.com/t/switching-workspaces-with-mouse-wheel-stopped-working/4112
Speculation by JanKusanagi in the #kde IRC chan is that this is due to the folderview widget kidnapping the mouse wheel. The behavior of switching desktops via mousewheel changed some versions ago earlier this year (running archlinux ~amd64). A downgrade to an older package plasma-desktop back from the days when it was (most likely) still working did not help though.

STEPS TO REPRODUCE
1. Configure >1 workspace, apply changes
2. Configure workspaces, change layout to "folder view" and ensure that vertical scrolling as well as middle click is set to "change workspace", apply changes
3. move the mouse to the desktop area and use the wheel to scroll vertically, afterwards use middle click

OBSERVED RESULT
Nothing happens when scrolling vertically, middle click opens the workspace selector as expected

EXPECTED RESULT
Switching workspaces or at least an information that this is not possible via wheel in folder view, middle click opens the workspace selector as expected

SOFTWARE/OS VERSIONS
Windows: N/A
macOS: N/A
Linux/KDE Plasma: 5.6.9-arch1-1, 64-bit
(available in About System)
KDE Plasma Version: 5.18.4
KDE Frameworks Version: 5.69.0
Qt Version: 5.14.2

ADDITIONAL INFORMATION
Same issue as described in this forum post: https://forum.endeavouros.com/t/switching-workspaces-with-mouse-wheel-stopped-working/4112