qml animation blocked

Hi:
I write a simple qml animation and run it on platform wayland,

I find the animation can't stop in the right place. The "shift" is about "8.177" when the animation stopped which the right value should be about "0". I think the last frame maybe skipped.
I run the qml on platform eglfs。 The animation stops rightly.
I enable QSG_RENDER_TIMING=1 and get the log in the attachment.
I read the code QSGAnimationDriver::advance(). I use the default vsyncmode. But my really fps is about 30fps.
My wayland client buffer count is 2. The QWaylandGLContext::swapBuffers will return soon. So the animation timer may think my fps is 60. I want to ask how to reslove this problem on platform wayland or some debug advices? Thank you.

I also try QSG_FIXED_ANIMATION_STEP=1, but it still have the same problem. I know Qt5.4.1 is change something about animation compared with Qt5.2.1. Does it affect?
The qml case:
import QtQuick 2.0
Rectangle {
id: root
width: 600
height: 600
color: "blue"
property real shift: 0
Rectangle

{ id: tt x:20 + shift y:20 width: 100 height: 100 color: "red" }

Rectangle

{ id:t2 x:140 - shift y:20 width: 100 height:120 color: "yellow" }

MouseArea

{ anchors.fill: parent onClicked: anima.start() }

SequentialAnimation on shift {
id: anima
SequentialAnimation {

NumberAnimation

{to: 120; duration: 100; easing.type: Easing.InOutSine }

NumberAnimation

{to: 0; duration: 100; easing.type: Easing.InOutSine }

loops: 1
}
}
Text

{ text: "shift: " + shift }

}