Small icons render poorly, look jagged when png (not svg)

Small icons (large-to-medium PNGs and not SVGs) turn out very jagged with the latest update to Qt 5.12.0 on Manjaro Linux with Plasma 5.14. This also occurs throughout Plasma, and is particularly visible in the notifications as well (notify-send --icon=/path/to/some/large/png content).

However, notice that the previews of the actual PNGs look perfectly fine.

This is unrelated to KDE Frameworks considering that KDE neon dev edition has the latest Frameworks with Qt 5.11.2 installed and this issue does not occur.

Sorry if my bug report's a little vague :/ I'm unfamiliar with Qt and I've never filed a bug here before. You can reproduce this bug by setting up Manjaro or Arch Linux with Qt 5.12.0 and resizing the icons, the rendering gets progressively worse as you turn down the icon size.