Try to reformat (Tools->QML/JS->Reformat File) the following test file:

import QtQuick 2.5
// Test
Item {
}

 The result is normal:

import QtQuick 2.5

// Test
Item {
}

But if you try to reformat again, the result will be:

import QtQuick 2.5


// Test
Item {
}

Per se the presence of two line breaks is not really serious.

But it is maybe a little bit more disturbing to have two different results after reformatting two times the same file.

Note: the same behavior is observed with all the types of comments (// /* /*!) after a block of imports.