I create a text box, change font size in the dialogue on top of the text box, add text, select all text - and then go to the fonts dockable window and try to change the font. Sometimes it works, sometimes it doesn’t, and sometimes only one line of text gets affected (while I have both lines selected)…
The tools options/general font selection apply to the default text. When you apply a change via the on-canvas dialog, the selected text is put in its own bubble and is isolated from the general settings. If you want to remove the bubble and put the text back under the general control, select the text and hit the Clear style of selected text button: