V4: Two variables assigned the same value ends up with two different values.

To reproduce, open the attached files in qmlscene.

The example is simplified as much as I managed. Inside a fairly complex loop, I do:
test1 = xx; test2 = xx;

At the end of the loop, test1 and test2 have different values.

This works as expected with Qt 5.6.2. I have reproduced the problem with 5.7.1 and 5.8.0.

Edit: This misbehaves with 5.6.2 as well, but there is some randomness. It also goes wrong with the latest 5.9 branch.