When outlook is offline, it puts Email in SENT folder rather than keep them in the Outbox

Question

One of the computers at the office has a weird issue with Outlook 2016. This only happens when there is no internet connection. When the laptop is offline, any emails written and sent in outlook will not stay in the outbox, outlook moves them to the sent folder even though there's no internet

Here's how to reproduce it:

1. Put the laptop on airplane mode / turn wifi off (Turn internet off)
2. Write an email, click send.
3. Outlook puts the email in the outbox
4. Outlook will then put the email in the SENT folder.
5. Turn the internet on, no emails will get sent

Outlook is connected to Gmail through Google sync (we're using G Suite for business).

Is there a way to fix this?

I have resync'd outlook, reinstalled outlook, and restarted the computer.