SetLinearVelocityY not working

Using the latest beta version of T2D I have the following code

function t2dSceneObject::onCollision (%srcObj, %dstObj, %srcRef, %dstRef, %time,
%normal, %contactCount, %contacts)
{
if ($player == %srcObj)
{
DeflectObject (%dstObj, $player.getPositionX ());
StunPlayer (%dstObj.type.stun);
}
if (%dstObj == $player)
{
DeflectObject (%srcObj, $player.getPositionX ());
StunPlayer (%srcObj.type.stun);
}
else if (%srcObj.moving && %dstObj.moving) // drop to drop collision
{ // average the speed of the two drops
DropCollision (%srcObj, %dstObj);
}
}

In my DeflectObject function I have

function DeflectObject (%obj, %deflectX)
{
if (%obj.getPositionX () == %deflectX)
{
// random deflection direction
%deflectX = getRandom (0, 1) * 60;
}

%obj.setLinearVelocityY (-0.7 * %obj.type.weight);
if (%obj.getPositionX () < %deflectX) // deflect to the left
{
%obj.setLinearVelocityX (-0.7 * %obj.type.weight);
}
else // deflect to the right
{
%obj.setLinearVelocityX (0.7 * %obj.type.weight);
}
}

My X velocity is appropriately applied, but the Y velocity is not (the object has an initial setLinearVelocityY (10) when it is created along with a max and a constant force: i.e.
%drop.setLinearVelocityY (%drop.type.weight);
%drop.setMaxLinearVelocity (%drop.type.weight);
%drop.setConstantForce ("0" SPC %drop.type.weight, true);

If I remove the constant force the object travels across the screen horizontally (instead of diagonally up).

Is this a bug in the beta code or am I doing something wrong?