Oli,
I viewed your test page on Chrome on Mac OSX and can see controls on both players when the overlay ad appears at 5 seconds. Mousing out of and back over both players allows the controls to hide and reappear.
Since these are Brightcove players, there isn't likely anything the Google IMA SDK team can do if there were an issue. If you were able to reproduce the issue outside of a Brightcove player, then perhaps this SDK team could help you.
Are you experiencing the issue on a particular OS, platform or device or can you provide a new link that does reproduce the issue? If so, I can work with our support team at Brightcove to help track down the issue.
-Mike Arguin
Software Engineer
Brightcove Customer Engineering