GP is using a Traefik reverse proxy, which is where the extra stuff comes from.

Plus, OP is using Collabora and caldav/cardav which needs some special consideration when reverse-proxying.

Both of those work out of the box for me on my reverse proxy. I use the built-in Collabora install though so maybe that's where the difference comes from.

I'd have to double check but I think I had to tweak some things regarding caldav (but it may have been years ago).