I setup a configuration file with the following presets entries:

  "presets": {
    "presets": {
      "petite pause (5 mn)": 300,
      "grande pause (12 mn)": 720,
      "séance travail (25 mn)": 1500
    }
  }

Notice the use of one accented character. The file is written in UTF-8 encoding. Now I restart gnome-shell. I see my presets, they are OK. I modify the duration in "Set Timer" to force a rewrite of the configuration file and now the configuration file is in ISO-8859-1 instead of UTF-8. I restart gnome-shell, the presets are no longer visible in the menu...