When using step.completed ? '1' : '0' for the x-model for steps Alpine was throwing a Invalid left-hand side in assignment error (although the code still worked on submission). Wanting to remove this error I instead set x-model to just be step.completed.

This then meant that when the form was submitted the type of completed was defaulting to a string boolean "true" or "false" and therefore wasn't passing validation. The only appropriate work around I've found was to use prepareForValidation within my IdeaRequest class and rebuild the steps array. Posting this here in case someone else encounters the same issue.

class IdeaRequest extends FormRequest
{
    public function authorize(): bool { ... }
    public function rules(): array { ... }

    protected function prepareForValidation(): void
    {
        if ($this->has('steps')) {
            $steps = [];
            foreach ($this->steps as $index => $step) {
                $steps[$index]['description'] = $step['description'];
                $steps[$index]['completed'] = filter_var($step['completed'], FILTER_VALIDATE_BOOLEAN);
            }

            $this->merge([
                'steps' => $steps
            ]);
        }
    }
}

Update Idea Action