I tested on my 720p panasonic tc-32LX60 and have the same horizontal sync issue only with the NESRGB.
I also tested my Emachines E211H 1080p PC monitor which shows the same issue in both x2 and x3 mode.
I tried running through the Sync Strike before the OSSC and it exhibited the same issue. I have not tried removing the Extron 128 from the chain.
I’m going to need to hook my gbs-8220 CFW Project back up and confirm that I don’t see anything similar. The NESRGB did take some different sync settings with the GBS than the rest of my 240p consoles.