Scaling settings apply differently to various applications depending on what framework/toolkit is used by specific application and depending on the value of the scaling factor. For some applications settings properly take effect before restart, but reset after.

Reproducible: Always

Steps to Reproduce:
1. Open "System Settings" -> "Display and Monitor" -> "Scale Display".
2. Set scaling factor to a number greater than one.
3. Start various Qt5, Qt4 and GTK applications.
4. Log out and then log in (restart).
5. Start the same applications again. Compare to the step 3.
6. Try again with different scaling factors (e.g. 1.4, 1.5 or 1.6)

Actual Results:  
Qt5 (e.g. Dolphin, Konsole), GTK (e.g. Firefox) and Qt4 (e.g. KolourPaint) applications are properly scaled only if scaling factor equals to 1.0 (no scaling), 1.5, 2.0, 2.5 or 3.0. Otherwise Qt5 applications are scaled partly (icons, controls are scaled but fonts are not) and GTK applications along with Qt4 applications lose all their scaling settings after restart (but they're properly scaled before restart). (URL of screenshots before restart with the scaling factor 1.9 and after restart with scaling factors 1.5 and 1.9 is attached)

Expected Results:  
All applications are properly scaled according to the settings. (somewhat like it happens in Windows or when the scaling factor is 1.5 or 2.0)

I used KDE Neon User Edition x86_64 (neon-useredition-20160725-1925-amd64.iso) in the live mode (KF 5.24, Plasma 5.7.2). Very similar problem occurred when I tried the KDE spin of Fedora 24 in the live mode (KF 5.22, Plasma 5.6.4). Computer is a laptop with Intel HD 5500 video card and 1920x1080 14" screen. The problem is also reproducible in VirtualBox (I tried Fedora 24 KDE).