That is indeed a known "issue", the better part of the actual svMotion is done by independent tasks in vSphere.
Running the Move-VM cmdlet with the RunAsync parameter and then monitoring the state of the background task, will not help either.
The only way that I know that will work is to launch the svMotion and then monitor the vCenter server for the VmMigratedEvent event.
And of course verify that you have the corresponding one with your svMotion.
Another, far fetched alternative imho, is to lower the vCenter settings for the number of parallel svMotion to 1.
Then you can fire off the svMotions like you did, but the system will handle them one by one.
See Frank's Limiting the number of Storage vMotions post.