BVM passthrough issue with 480i from PAL GC

[Coestar]

I have a bit of a complicated daisychain that I use for playing and streaming older consoles. Here is the normal setup:

CONSOLE -> RGB JP21 out -> JP21 to BNC -> BVM RGB in -> BVM RGB out -> BNC to SCART -> SCART to Framemeister RGB in -> Framemeister HDMI out -> HDMI in capture card

Normally, at least with 240p signals, this works without issue. It allows me to play on the BVM without input lag while simultaneously capturing/streaming high quality video. However, I've recently added a PAL Gamecube (unmodded) to my setup and it has uncovered an issue. Whenever I attempted to play games that were running at 480i, the output (after BVM) would suffer an intermittent flicker. Here's a video of it happening: https://www.instagram.com/p/BEwadiSi7Ag/ (notice the issue shows only on the LCD at the top, the BVM is unaffected) 240p signals sent from the PAL GC through the BVM don't have this issue, only 480i.

After eliminating as many devices as I could from the daisychain, I've narrowed the issue down to the BVM's RGB out. Something happens to the 480i signal when it's output from the BVM, but I don't have a clue yet exactly what. Any ideas?

[Guspaz]

Have you tried a different cable for the BNC to SCART or the SCART to Framemeister cables?