Hello 
This happened with me recently with js.badges.awarded string. My site is in Hungarian language.
When the js.badges.awarded was implemented I translated it on /admin/customize/site_texts and when the pluralized version is overrided it the not pluralized version js.badges.awarded was stucked and was not findable on /admin/customize/site_texts.
Because the available strings on /admin/customize/site_texts was only the js.badges.awarded.one and js.badges.awarded.other
I am not sure why it’s happened but maybe because the js.badges.awarded was not contains interpolation keys and the new pluralized one contains the %{number}. 
So it seems if the text is overridden then the updated version will not replace this and the previously overridden text will keep.
I removed this in rails.
TranslationOverride.where(translation_key: "js.badges.awarded").delete_all
By the way it’s usually works like if a string is translated and then the pluralized version comes then on Hungarian language it use the other format.
I am not sure it’s a bug? And that it happens with the pluralized version it’s maybe can happen without pluralized change.
Thank you 