Hi Andy
I have the same problem.
My setup is as follows:
Cable Receiver (UPC Switzerland, EOS BOX) => HD Fury Integral2 => Marmitek Connect HA13 (HDMI2RCA) => PureLink Grabber => RPI 3+ => LEDs
As long as the receiver is on, all is working fine. But as soon as i turn of the receiver, i only got a blue picture. Grabbing this with hyperion screenshot shows me, that this is somehow a PAL testpicture. I figured out, that the HA13 converter is sending this out as soon as he didn't receive anymore a signal..
does someone has the same situation?
or can someone recommend a converter with another behavior?
of course, i can poll my tv (are you running on HDMI1?) and then enable/disable hyperion with a scrip.. but.. 
I'm searching for a more elegant way for solving this.
thanks for any hint!
bes regs
neil