My setup is a Radeon R9 390X running 18.3.2 drivers on Windows 10 Pro x64, connected to a PC monitor via DVI, and to a Pioneer Elite SC91 AV receiver through HDMI that I use when I want to display on my TV.
This setup was working great both in clone mode and extended desktop for a long time. This week I upgraded the TV connected to the receiver to a Sony XBR55x900E. Now I have an issue where my PC will be fine when just displaying on my PC monitor alone, and when the receiver is switched to the HDMI input my PC is connected to for multi monitor mode. However, if I swap the receiver to a different HDMI input for TV/Xbox/whatever else, my PC constantly switches from single monitor mode to dual mode, and back and forth every 2-3 seconds. Upon changing the receiver input back to the PC it goes back to working great.
I've tried turning off HDMI-CEC in case the new TV was giving it issues but it makes no difference.
Since the only difference is the TV connected to the receiver I wouldn't think that would cause a problem like this. Any suggestions?