Using jsforce, after negotiating the OAUTH2 flow, obtaining refresh & access token, but can't use them

Question

After placing the refresh token, access token and the instance URL in the session, I have the following code:

  app.get('/api/v1/refresh', function (req, res) {
    const conn = new jsforce.Connection({
      oauth2: {
        clientId: process.env.SALESFORCE_CONSUMER_ID,
        clientSecret: process.env.SALESFORCE_CONSUMER_SECRET,
        redirectUri: process.env.SALESFORCE_CALLBACK_URL
      },
      instanceUrl: req.session.user.salesforce.instanceUrl,
      accessToken: req.session.user.salesforce.accessToken,
      refreshToken: req.session.user.salesforce.refreshToken
    })

    conn.oauth2.refreshToken(req.session.user.refreshToken, (err, results) => {
      if (err) {
        return res.json({ error: err.message })
      }
      res.json(results)
    })
  })

I'm aware you don't have to refresh with jsforce, but I'm just trying to see the tokens work. In this case, I get the error: invalid_grant: expired access/refresh token - but the token is only seconds old?

Using conn.sobject, conn.query or conn.search results in no callback being called.

The scopes setup for the OAUTH2 flow are:

• API
• profile
• web
• refresh_token, offline_access

Thank you in advance for your help.