Erasing with backspace key while using an input method leaves a spurious character

How to reproduce:

1. Ensure an input method is enabled. Any input method will do, and note that in GNOME since Fedora 29 diacritics in Spanish (and probably in many other European languages) are now handled through an input method.
2. Press a key that produces a temporary character. For instance, a diacritic mark in Fedora 29 will do. Alternatively, you can use e.g. Anthy (Japanese input) and type a 'k'.
3. (Optional, not relevant for European language diacritics) You can keep typing and it will work fine. For instance, in the Japanese test case you can press 'a', 'k' and 'u' to produce 'かく'.
4. Now, press backspace to attempt to remove the temporary characters, cancelling the input. In the Japanese test case you need to press backspace twice (once to remove 'く' and again to remove 'か').

Expected result:

The text should look exactly like at the beginning, since we cancelled the input. This is what you would expect in any application, and is the behavior you can find in a normal QLineEdit for instance.

Actual result:

The last temporary character (either the lone diacritic or the 'か') remains visible in the editor like another character even though it's not really part of the file if you save now. After this the editor gets very buggy and you need to close the file and open it again.