I had the same issue in the latest AE version (16.1.1). I googled it, but failed to find an answer.
Fortunately, I found a simple workaround for this issue. You just need to explicitly make an element active, then make it inactive back. In your case it might look like:
myPanel.grp.groupOne.myPurgeButton.active = true;
myPanel.grp.groupOne.myPurgeButton.active = false;
It might be useful for those, who stumbled across the same problem.
All the Best,
Konstantin Nemo
Music Visualizer Templates