In T8177#178194, @ngraham wrote:

You should use the Kubuntu backports PPA to upgrade to Plasma 5.15 and see if that fixes the issue. https://launchpad.net/~kubuntu-ppa/+archive/ubuntu/backports

If it's still broken in Plasma 5.15, then you should file a bug. :) https://bugs.kde.org/enter_bug.cgi?product=plasmashell

Please make sure to explicitly state that you are using PLASMA_USE_QT_SCALING=1 for the purpose of finding bugs with that setting.

In T8177#178195, @dos wrote:

I've checked on my PC with the same screen and 5.15.2. It works fine with 2x factor (except the Magic Lamp, which I already mentioned in my last comment - haven't filled a bug yet, will do so soon), but shows similar symptoms when using fractional factors.

@dos
2x as in Scaling factor in System Settings or setting PLASMA_USE_QT_SCALING=2?

In T8177#157609, @acrouthamel wrote:

I just wanted to mention this still works fine for me and resolves numerous icon scaling issues.