How to reproduce on iOS:

1. Open the example "Qt Quick Controls - Text Editor"
2. In texteditor.qml remove "footer:", so the toolbar is not covered by the keyboard
3. Start the app on iPad 
4. Start a new line
5. type in a word, e.g. "Safa"
6. Make it Bold
7. Use predictive typing to complete the word to "Safari" (or any other word)
8. The completed text has default font

On Android the behavior is a bit different, but also buggy. I can complete Safari without losing the formatting, but when pressing backspace predictive typing kicks in and the formatting is lost.