SUMMARY After using the highlighter tool (as well as the underline tool, etc.) once, it automatically unselects itself and selects the "text selection" tool. Thus, in order to highlight again, the user needs to select again the proper tool. EXPECTED RESULT It would be more efficient of the highlighter tool (as well as the underline tool, etc.) remain selected throughout, until the user wishes to use another tool. This is the default behavior in other PDF editors (e.g. PDF Studio, Adobe Acrobat, etc.).
Read the manual. "If you e.g. want to highlight all important parts of a text, activate that tool permanently by double clicking on the tool button. " https://docs.kde.org/trunk5/en/kdegraphics/okular/annotations.html