Hieroglyphs instead of Cyrillic in Flutter application, how to fix?

Question

JavaScript

In a Flutter application, the design of the main screen is configured through the config.json file located on the hosting.
Connecting the file via constants.dart:

Code snippet in the config.json file itself:

How it looks in the application:

As you can see from the 3rd screenshot, the localization works fine (the ru.arb localization file and the generated i18n.dart). Even on the same main screen, the Russian language is displayed normally, but for some reason, hieroglyphs come from this file. The file encoding is in UTF-8 like all other application files. What could be the problem?