I long ago replaced the clocked video input block with my own.
I would suggest two things:
1 - What type of video are you bringing into the clocked video input. If you take the video away, does it continue to receive input? Meaning, would the clocked video input block have any knowledge that the video was taken away? If it stops receiving a clock or a data valid signal (depending on the type of video) it may just be waiting for more data and not knowing that it actually lost a signal.
2 - Are there any changes to the other status bits (bits 1 - 7) of the status register when you've lost video?
Jake