BFI causing strange flicker/pulsing with arcade roms

First and foremost consider this:

• Only RetroArch bugs should be filed here. Not core bugs or game bugs
• This is not a forum or a help section, this is strictly developer oriented

Description

Black Frame Insertion appears to have pulsing/flickering (not the normal, expected flicker) that happens when running various arcade games using any working core (FB Alpha, FB Neo, MAME, etc.).

This takes place at BFI 1 (120hz) and BFI 2 (180hz) using either GL and VULKAN back ends.

I've verified it happening on 2 separate systems, both very powerful; 5800x with a 3080, and 5600g (onboard APU). BFI works perfectly fine on all console systems (SNES, NES, GENESIS, etc.)

Expected behavior

BFI should work properly on all cores/systems

Actual behavior

BFI causes random, large flickers/pulsing when loading many arcade games. Specifically, I tested Final Fight and Double Dragon with a number of cores, all producing the same issue.

Steps to reproduce the bug

1. Set refresh rate to either 120hz or 180hz on desktop
2. Launch retroarch and set the gpu driver to either VULKAN or GL
3. Select BFI 1 or BFI 2 (depending on 120hz or 180hz monitor refresh)
4. Load any 'arcade' core and try to play Final Fight/Double Dragon

Bisect Results

[Try to bisect and tell us when this started happening]

Version/Commit

You can find this information under Information/System Information

• RetroArch: e954a46

Environment information

• OS: Windows 11
• Compiler: N/A

Reproduce the issue using BFI 1 at 120 Hz and BFI 2 at 180 Hz with the GL and Vulkan video drivers, then compare arcade cores such as FB Alpha, FB Neo, and MAME against the listed console cores. Test Final Fight and Double Dragon on Windows 11 and identify whether the pulsing is shared by the video presentation path or specific to arcade content. Done means BFI no longer produces the reported abnormal flicker while the existing console behavior remains intact.