Delighted to say, finally, after intensive ongoing testing & investigation, it is solved. All thanks goes to my old unfediversed #Penguinista online friend of several years. 🤗​

• Plasma systems with 'khelpcenter' already installed, & then with #GnuCash [& its associated docs pkg] subsequently installed, suffer this symptom. That was my Host system.
• Plasma systems without 'khelpcenter', & then with #GnuCash installed, have correctly working GC Help docs.
• To fix the problem, one can simply uninstall 'khelpcenter', & thence tolerate future KDE apps' Help opening in one's browser rather than the native #KHelpCenter windows, as the price to pay for then having functional GTK apps' Help opening in native windows.
• Given i run Plasma not Gnome, that seemed an absurd price to pay, so the following compromise was reached.
• Edit '/usr/share/applications/mimeinfo.cache', find the four lines containing 'yelp.desktop', note that three of them will have this value, 'org.kde.khelpcenter.desktop;yelp.desktop;', edit them to place 'yelp' first, ie, 'yelp.desktop;org.kde.khelpcenter.desktop;'.
• Cake, eat [future Plasma updates probably will return these to default sequence, so this fix might need reapplying after each such system update]. Note this fix does incur a small price; whilst immediately after the edit, KDE apps' Help will still open in KHelpCenter windows & GC Help will open in GTK [ie, #Yelp] windows, after reboot all Help, KDE & GTK, will open in Yelp windows.

#KDEPlasma #GnuCash