Pressing enter on autocompletion suggestions does not result in them being inserted

I'm using a self-built Creator on the 4.14 branch (5a9a9e800cd793bbc8607eee93023c8cda0fad9b) to test out some Qbs stuff. I noticed that when choosing autocompletion results with enter, sometimes the suggested text doesn't get inserted into the code. It seems that this message in Creator's output coincides with that:

SOFT ASSERT: "m_proposal" in file ../../../../qt-creator/src/plugins/texteditor/codeassist/codeassistant.cpp, line 369 

I haven't noticed this with 4.13.x.

There's a video attached that shows the problem:

creator-auto-completion-4.14.mp4

At the end I press enter to use the suggestion, but nothing happens besides the context menu closing. Notice the extra line in the console output afterwards.

So far, this has been reproducible every time I've tried. However, I noticed that, after it fails, if I then press Ctrl + Space after "target" to get context menu to come up again, pressing enter then results in the suggestion being used, as expected. So it's only an issue when writing the line from scratch in this particular case. In other cases doing a similar thing (bringing up the context menu in the middle of a line) results in the same problem.