Just tried it on Geany 2.0. Same effect. Font does not change. I tried the change from the font Jet Brains Mono Medium to the font Jet Brains Mono Light Italic (on the grounds that if I change to an italic font, the change would be immediately visible). In the status bar, I get the message
14:49:51: Font updated (JetBrains Mono Light Italic Condensed 17).
but the font itself does not change.