Hi,

in my case, the title bar has a button in the top right corner for closing the window, but no buttons for minimizing / maximizing. In addition, when I right-click the title bar, the options for minimization and maximization are grayed out.

The screenshot on https://apps.kde.org/en/kfind shows the additional buttons and hence suggests that minimizing / maximizing is possible.

I have no such problems for other KDE programs such as Kate, KRename or KDiff3.

KFind Version: 4:19.12.3
Xfce Version: 4.14
Ubuntu Version: 20.04

Works fine for me on Kubuntu 20.10 with Plasma. Maybe an update is a good idea... keep in mind that LTS distribution are mainly targeted at businesses and mass deployment, the regular releases are more suitable for private users.