Incorrect Resource URLs message when they are correctly set

We have recently upgraded to CiviCRM 5.13.4 on Drupal 7 and quite a lot of sites have the issue where the 'Incorrect Resource URL' message is showing despite them being set correctly.

Most of the sites are just the default paths so I find it hard to believe that there is problems on all of them.

Pre-upgrade, the websites were not showing the message.

This doesn't seem to be an isolated incident as another member of the community has also said that he had the same issue.