Adjusting font sizes by trial-and-error is difficult because the font selector selects the first font on the list upon instantiation (instead of the currently configured font)
In the prefs dialog with version 43, each time you click the font button to adjust the font size after applying and trying it out with "real text" across the whole text editor window, the font selector preselects the first font in the alphabetical list (ex: DejaVu or whatever may come first, instead of "Droid Sans Mono" or "Libertinus" or Inconsolata" or whatever you may have picked), so it effectively resets your font family choice without telling you (it's not visually clear that it's doing so).
Instead, gnome-text-editor should tell the font chooser to preselect the currently configured font, not just set the font size.