SUMMARY
When invoking Overview (Meta+W), an edge of the screen will contain the "Desktop Bar", with previews of Virtual Desktops that allow dragging individual windows to them, controls to add and remove virtual desktops, and a search filter. 

Since Plasma 6.0, this bar only displays when the Virtual Desktops are aligned in a row or column, such as 1x4 or 6x1, (not 2x2 or other square layouts). In 5.27, the bar was always visible in Overview no matter the layout. 

STEPS TO REPRODUCE
1. In fresh Plasma session, define 4 virtual desktops in 1 row, 
2. Open Overview and observe desktop bar on the top edge.
3. Adjust rows in System Settings -> Window Management -> Virtual Desktops

OBSERVED RESULT
4 desktops in 2 rows (2x2), the Desktop Bar disappears. 
4 desktops in 4 rows will show the desktop bar on the left edge.

EXPECTED RESULT
Desktop bar should always be visible in Overview on a screen edge.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch / Plasma 6.0.5

ADDITIONAL INFORMATION

This seems indirectly caused by changes to the Virtual Desktop system in Plasma 6, https://bugs.kde.org/show_bug.cgi?id=482418 seems related, but this is distinctly an issue with the Desktop Bar. 

I see places in DesktopBar.qml that I'd like to try and test to see if this can be quick-fixed, but I have a feeling this will require a deeper look at the Virtual Desktop code.