Rounded corners gives large frames

[gforcedev]

• Arch linux, dwm (no patches)
• Nvidia drivers (mx150), installed with the arch install script
• Latest version of this fork installed from https://aur.archlinux.org/packages/picom-rounded-corners/

Configuration:

experimental-backends = true
backend = "glx"
shadow = true
shadow-offset-x = -12
shadow-offset-y = -6
shadow-radius = 12
shadow-opacity = 0.30
inactive-opacity = 0.7
frame-opacity = 0.0

corner-radius = 0
opacity-rule = [
  "60:class_g = 'St' && !focused"
];

round-borders = 1
round-borders-rule = [
  "3:window_type = 'unknown'",
  "3:window_type = 'toolbar'",
  "3:window_type = 'utility'",
  "3:window_type = 'splash'",
  "3:window_type = 'dialog'",
  "3:window_type = 'normal'"
];

blur: {
  method = "dual_kawase";
  strength = 10;
  background = false;
  background-frame = false;
  background-fixed = false;
}

Steps of reproduction

Ideally I'd want to add corner-radius = 12 into here instead, but when I do I get thick frames / borders (thicker than frame-opacity = 1) - blue around the focused window, grey around the others.

Apologies if this just needs a config option I'm not familiar with, but it feels like a bug. Any help would be appreciated.