Theme export only saves default colours

When saving a theme colour changes are not exported, regardless of the 'keep' settings.

Here's the configuration I wanted to save:

Having added

/opt/pleroma/instance/static/themes/styles.json

{
  "pleroma-dark": "/static/themes/pleroma-dark.json",
  "pleroma-light": "/static/themes/pleroma-light.json",
  "pleroma-amoled": [ "Pleroma Dark AMOLED", "#000000", "#111111", "#b0b0b1", "#d8a070", "#aa0000", "#0fa00f", "#0095ff", "#d59500"],
  "classic-dark": [ "Classic Dark", "#161c20", "#282e32", "#b9b9b9", "#baaa9c", "#d31014", "#0fa00f", "#0095ff", "#ffa500" ],
  "bird": [ "Bird", "#f8fafd", "#e6ecf0", "#14171a", "#0084b8", "#e0245e", "#17bf63", "#1b95e0", "#fab81e"],
  "ir-black": [ "Ir Black", "#000000", "#242422", "#b5b3aa", "#ff6c60", "#FF6C60", "#A8FF60", "#96CBFE", "#FFFFB6" ],
  "monokai": [ "Monokai", "#272822", "#383830", "#f8f8f2", "#f92672", "#F92672", "#a6e22e", "#66d9ef", "#f4bf75" ],

  "redmond-xx": "/static/themes/redmond-xx.json",
  "redmond-xx-se": "/static/themes/redmond-xx-se.json",
  "redmond-xxi": "/static/themes/redmond-xxi.json",
  "breezy-dark": "/static/themes/breezy-dark.json",
  "breezy-light": "/static/themes/breezy-light.json",
  "mammal": "/static/themes/mammal.json",
  "paper": "/static/themes/paper.json",
  "mytheme": "/static/themes/mytheme.json"
}

and the json file containing downloaded preset, then set the default name as my theme for the site in admin/#/settings/frontend -> Theme

This is the result:

and the content of the exported theme if not saved in the output...