Application stuck in maintenance mode at one specific location

I believe this is a server/network related issue, so decided to post here. Please let me know if this is not appropriate and should be moved.

So here's what happened:

• Over the weekend we migrated our application to a new server. Before everything was done and tested we had our app set in maintenance mode on both the old and new server.
• After the migration was done and DNS resolved to the new server I ran php artisan up on the new server. This brought the application back online. So far so good!
• Yesterday we get a phonecall from a customer saying he couldn't reach the application. He tried multiple browsers on multiple devices in his home. Apparently he was receiving a 503 response indicating our application was in maintenance mode.
• I have now enabled and disabled maintenance mode again on our application + cleared the view cache, as a sort of sanity check, and like magic, the application started working again for our customer...

TLDR: It appears our application was stuck in "maintenance mode" for one particular customer. This occurred on multiple devices and browsers, so there was no local caching involved.

What could be a reasonable explanation as to what might have caused this to happen?