Sharepoint - scripts not fully loading on custom login page

Question

First question here, hope someone can help. We have a SharePoint 2019 Server extranet hosted in Azure that uses a custom login page for Windows and form-based authentication. The .aspx page was modified last week directly on the web front end servers and the login page now appears blank when loaded.

The browser console shows that init.js has not completely loaded, only a portion of the script appears when viewed in the browser. The portion is always the same length. This causes the page not to render and produces other javascript errors ('RegisterSOD not defined'). The init.js file on the web servers is complete and hadn't been modified.

However the login page displays correctly when I access the sites directly from the web server (ie by changing the hosts file entry to point to the server's IP address). Users that are are already logged into the site are able to continue using it, but get the blank page login page if they log out of their session and try to log in again.

A bit more about our Azure environment - we have two load balanced WFEs behind two web application proxies which have their own load balancer.

Any idea on where to start? It feels like a caching issue (blob caching isn't enabled by the way) but I don't know where to look first.