I have figured out what was wrong:

Since I am using asdf, I have removed the following lines from my .zshrc file:

export PATH="$PATH":"/Users/myusername/.asdf/installs/flutter/3.22.2-stable/bin"
export PATH="$PATH":"/Users/myusername/.asdf/installs/flutter/3.27.1-stable/bin"

I have uninstalled every version of Dart from asdf.
In my workspace settings I have removed the dart.sdkPath setting entirely.

The problem was that the stand-alone Dart SDK does not include Flutter, where the Flutter SDK does include the Dart SDK, therefore if using Flutter then the Dart SDK should not be referenced or even installed separately.