This sounds like something very artificial but the steps to reproduce this bug could be a year apart.
Assuming your forum is set to language A to start with.
- Edit a text in language A using Customize - Text
- Change the default locale of the forum to B
- Hard refresh your browser
- Go back to Customize - Text
- Search for the text you have just changed in step 1. The dropdown will be on language B since it is the default locale.
- You will find the text you’ve changed but there is no way to see that this text was found in language A and not in the currently selected language.
- Click Edit
- The edit box will show the text of language B since that was what was in the dropdown.
The only way to figure out that this text was found in language A is to look in the database, or to try to set the dropdown to different languages until you find it.
Long story short: the Edit buttons should use the language of the overridden text, not the language in the dropdown.