This is hard to reproduce, as it seems to only happen after Creator has been open for a while (restarting Creator and following the same steps as shown in the GIF does not reproduce the issue). The problem is always the same though: using an auto-completion suggestion overwrites text on that line with the suggestion contents, plus the text that was typed to prompt the suggestion.

Reproduced on a fresh install of Ubuntu 16.04 (installed yesterday). I can't remember if it has happened on any other platforms.