ISO-UK keyboard only maps correctly when Karabiner is set to ansi

[sbaildon]

{
   // ${XDG_CONFIG_HOME}/karabiner/karabiner.json
   "virtual_hid_keyboard": { "keyboard_type_v2": "ansi" }
}

My physical keyboard is ISO-UK. I've a short left-shift, whose right neighbour is ` + ~. Using the macOS keyboard identification tool in System Settings/Keyboard, the keyboard is identified as ISO — this is totally as expected.

However, if I have karabiner set to ISO, my ` + ~ key becomes § + ±. I have to explicitly go against what makes sense and change my virtual keyboard to ansi for the logical mapping to reflect the physical keyboard.

This doesn't seem normal. Am I doing something wrong?