i’ve always just selected the font files in dolphin from downloads or wherever and dragged them into the font manger window.
even tho the names might appear in the font manager, you still have to at least logout for the fonts to become available in your apps.
if you can’t even get the names to appear in the font manager, check to see if you have the right permissions on the font files…some times they come over locked and i’ve needed to change them.
ah, i’ve never done it that way, but i tried it just now with a random font download and it worked
only i had to reboot in order to see the font listed in the font management tool, where if i drag and drop it would be listed immediately (iirc) even tho i still needed to reboot before i could use it in an app.
i would suggest filing a bug report using the help menu in the font view app.