We have encountered some odd behavior in the auto completion.
It appears that arguments with a dash as the second character are detected as flags by mistake.

The full reproducer with various test cases can be found here:
https://go.dev/play/p/UxqRkMcEX2a

The minimal reproducer is the following:

package main

import "github.com/spf13/cobra"

func main() {
	root := &cobra.Command{
		Use: "root",
	}
	root.SetArgs([]string{"__complete", "1-ff00:0:1", ""})
	root.Execute()
}

which results in the following output:

[Debug] [Error] Subcommand 'root' does not support flag '1'
:0
Completion ended with directive: ShellCompDirectiveDefault

I tracked down the issue and appears that we enter this branch by mistake:

In particular, it appears that the is isFlagArg classifies the argument as a flag byistake: