All the other fields work as intended, except for the text setting, which seems to do nothing. Why? How to fix this?
Using GIMP 2.8.18.
Once you have changed anything (including the font) on some characters using the on-canvas dialog, these characters are supposed to have their own formatting and won't be impacted by the Tool options settings.
The accepted answer is correct; if you have applied formatting to any of the text by using the little on-canvas floating dialog, then the affected text will no longer respond to settings in the "Text Tool Options" or "Font" dialogs.
But there is now an easy way around it!
Select the affected text, and click the "Clear Formatting" button (as per the image below) and the text will again be responsive to the general Text Tool Options & Font dialog settings.