Length issue with Javascript carry-forward choices?

I am using a matrix drag and drop on Qualtrics to ask respondents to sort 46 statements into the categories: agree less, neutral, and agree more (QID1).
QID1
In QID6 I ask them to sort the statements carried forward from the category agree less into -5 to 0.
QID6
Then in QID7 I ask them to sort the statements carried forward from the category agree more into 0 to 5.
QID7
In QID4 I show them how they sorted the statements (from -5 to 5) from QID6 and QID7. However, I wanted to combine how they sorted the responses in QID1, QID6, and QID7 specifically for the neutral and 0 responses. However, I have been unsuccessful at getting all the statements to carry forward. If only one statement was put into each of the categories of neutral (in QID1), category 0 in QID6, and category 0 in QID7 then those three statements appear in QID4.(QID1selection, QID6selection, QID7selection, result)
However, if more than one statement is placed into any of those categories (neutral in QID1 or 0 in QID6 and 7), then none of them appear in QID4. This does not seem to be an issue for the statements selected in the other categories (-5,-4,-3,-2,-1,1,2,3,4,5). Is this an issue with length or is concat the incorrect function to use? I am not familiar with javascript at all, so I am having a hard time figuring out what is the issue.
Below is the Javascript function I am using (based on this discussion post: https://community.qualtrics.com/XMcommunity/discussion/14728/carry-forward-response-into-drag-drop-box)
Qualtrics.SurveyEngine.addOnReady(function () {
let ques = this;
//let a1 = "${q://QID1/ChoiceGroup/SelectedChoicesForAnswer/1}";
let a2 = "${q://QID1/ChoiceGroup/SelectedChoicesForAnswer/2}"; //this is neutral
//let a3 = "${q://QID1/ChoiceGroup/SelectedChoicesForAnswer/3}";
let b1 = "${q://QID6/ChoiceGroup/SelectedChoicesForAnswer/1}";
let b2 = "${q://QID6/ChoiceGroup/SelectedChoicesForAnswer/2}";
let b3 = "${q://QID6/ChoiceGroup/SelectedChoicesForAnswer/3}";
let b4 = "${q://QID6/ChoiceGroup/SelectedChoicesForAnswer/4}";
let b5 = "${q://QID6/ChoiceGroup/SelectedChoicesForAnswer/5}";
let b6 = "${q://QID6/ChoiceGroup/SelectedChoicesForAnswer/6}"; //this is 0
let c1 = "${q://QID7/ChoiceGroup/SelectedChoicesForAnswer/1}"; //this is 0
let c2 = "${q://QID7/ChoiceGroup/SelectedChoicesForAnswer/2}";
let c3 = "${q://QID7/ChoiceGroup/SelectedChoicesForAnswer/3}";
let c4 = "${q://QID7/ChoiceGroup/SelectedChoicesForAnswer/4}";
let c5 = "${q://QID7/ChoiceGroup/SelectedChoicesForAnswer/5}";
let c6 = "${q://QID7/ChoiceGroup/SelectedChoicesForAnswer/6}";

let d1 = a2.concat(b6,c1); //this should combine all the statements selected for QID1 (ans2), QID6 (ans6), and QID7 (ans1)

let choice_text = ques.getChoiceContainer().innerText.split(/\\n/gm);
let all_choices = ques.getChoices();
let all_answers = ques.getAnswers();

for (let i = 0; i < choice_text.length; i++) {
let set_choice= -1
switch (true) {
case b1.includes(choice_text[i]):
set_choice = all_answers[0];
break;
case b2.includes(choice_text[i]):
set_choice = all_answers[1];
break;
case b3.includes(choice_text[i]):
set_choice = all_answers[2];
break;
case b4.includes(choice_text[i]):
set_choice = all_answers[3];
break;
case b5.includes(choice_text[i]):
set_choice = all_answers[4];
break;
case d1.includes(choice_text[i]): //this should show all the statements selected for QID1 (ans2), QID6 (ans6), and QID7 (ans1)
set_choice = all_answers[5];
break;
case c2.includes(choice_text[i]):
set_choice = all_answers[6];
break;
case c3.includes(choice_text[i]):
set_choice = all_answers[7];
break;
case c4.includes(choice_text[i]):
set_choice = all_answers[8];
break;
case c5.includes(choice_text[i]):
set_choice = all_answers[9];
break;
case c6.includes(choice_text[i]):
set_choice = all_answers[10];
break;
}
ques.setChoiceValue(all_choices[i], set_choice, true);
}
});
I would appreciate it if anyone can point me to the right direction or to any helpful resources.