There is nothing in express that is adding something called _locals that I am aware of. Can you provide a reproducible example of the issue you are having? Does it happen with all template engines or only ejs?

That was a fast reply and an unexpected answer.
Sure i"l try to provide anything i can.
No i haven't used or tried anything but EJS.

Here is the code:

groupby(Function)

const groupBy = function (xs, key) {
    return xs.reduce(function (rv, x) {
        (rv[x[key]] = rv[x[key]] || []).push(x);
        return rv;
    }, {});
};

Route

app.get('/test', async (req, res) => {
t = await db.getQuestions(1);
zz = groupBy(t, 'question');
res.render('pages/test', zz);
});

View(EJS):

<% for( const property in zz ) { %>
<p><%= property %></p>
<% } %> 

As far as the Data, this is how it looks like after each step:

From DB

[
  {
    question_id: 1,
    question: 'Lorem ipsum dolor sit amet',
    answer: 'Lorem',
    value: 1
  },
  {
    question_id: 1,
    question: 'Lorem ipsum dolor sit amet',
    answer: 'ipsum',
    value: 3
  },
  {
    question_id: 1,
    question: 'Lorem ipsum dolor sit amet',
    answer: 'dolor',
    value: 5
  },
]

After groupBy:

{
  'Lorem ipsum dolor sit amet': [
    {
      question_id: 1,
      question: 'Lorem ipsum dolor sit amet',
      answer: 'Lorem',
      value: 1
    },
    {
      question_id: 1,
      question: 'Lorem ipsum dolor sit amet',
      answer: 'ipsum',
      value: 3
    },
    {
      question_id: 1,
      question: 'Lorem ipsum dolor sit amet',
      answer: 'dolor',
      value: 5
    }
  ],
}

In the FE:

<p>Lorem ipsum dolor sit amet</p>
            
<p>_locals</p>

BTW if i stringify the object on render
res.render('pages/test', JSON.stringify(zz));

I get the following error:
(node:337233) UnhandledPromiseRejectionWarning: TypeError: Cannot create property '_locals' on string

So i'm thinking render adds that property on any object it sends.

Update:
Just tried putting that object in an object when rendering and that made the problem go away
res.render('pages/test', {zz});

Very strange but somehow related to passing an object directly to render rather than inside an object.
Never had a problem before...