It has a light background, like it's a tiny floating window. It does not have the typical black background used by tooltips that come from PlasmaComponents2, QQC2 desktop style, or Breeze QWidgets desktop style. This is inconsistent and should be fixed.
A possibly relevant merge request was started @ https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/96
Git commit b532fd2f2087ffaee65258a4f04c05970670077c by Nate Graham. Committed on 28/09/2020 at 14:44. Pushed by ngraham into branch 'master'. Make PlasmaComponents3 Tooltips use the typical tooltip style Without this, the tooltips are light when using a light theme. This is contrary to other tooltips, which are always dark no matter which coloration of theme you're using. FIXED-IN: 5.75 M +33 -11 src/declarativeimports/plasmacomponents3/ToolTip.qml https://invent.kde.org/frameworks/plasma-framework/commit/b532fd2f2087ffaee65258a4f04c05970670077c