SUMMARY
Some system settings text appears blocky when it is opened on a scaled display, until the window is moved to a display that is not scaled and then moved back again, which displays text with the correct aliasing. This does not affect all systems settings panels, it does not affect Proxy settings or Input Devices settings for example, but seems to affect most of what I tested (Everything under Appearance, Workspace, and Personalisation, as well as About this System).

Blocky text on 4k@150% scaled display: https://i.imgur.com/dQ0lzFs.png
After moving to 1080p display that is not scaled and back again: https://i.imgur.com/W0inwwT.png

This affects the Shortcuts menu in System Settings but it does NOT affect the shortcuts menu inside of other KDE applications, i.e. Spectacle. They render correctly. Also, I believe this issue is present even on 5.24, but I don't think it was quite as prevalent. At least if it was, I did not notice it in as many places in System Settings as I do now, but I can't verify this.


STEPS TO REPRODUCE
1. Open System Settings on a scaled display
2. Open one of the sub menus - For example, About this System
3. If text is blocky, move it to a non-scaled display

OBSERVED RESULT
Text is blocky in some System Settings submenus until it is dragged to an unscaled display and dragged back


EXPECTED RESULT
Text should display properly on scaled display without needing to be dragged to an unscaled display.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.17.9-zen1-1-zen (64-bit)
KDE Plasma Version: 5.24.90
KDE Frameworks Version: 5.94.0
Qt Version: 5.15.4

ADDITIONAL INFORMATION
- I am using a setup with 4 displays, three unscaled 1080p displays and one 4k display scaled at 150%. This could perhaps be related. With other similar issues such as Bug 448556, scaled text issues seem to go away when there is only one monitor. So the issue could be rooted more specifically in mixed-DPI or mixed-scaling display setups.
- Possibly related: Bug 448567, Bug 448556