If a JSON string contains emojis mixed with cyrillic letters, then in most cases emojis aren't displayed correctly after parsing JSON with QJsonDocument. In Qt 5.6.2 it doesn't happen.

The issue can be reproduced in the attached test project.

Edit: I've just found out that it's impossible to enter emojis in QtCreator 4.2.1 using Cmd+Ctrl+Space (seems related), and that pasting content of JSON file from the test project directly in QtCreator 4.2.1 results in the same broken emojis in strings with cyrillic:

Running the test project with strings directly in code produces the same results as with reading from JSON. More looks like a rendering bug.