Ubuntu Software Center doesn't work with local/mirrored repositories

Ubuntu Software Center doesn't work with local/mirrored repositories.

I mirror the entire Ubuntu repositories to a hard drive every month to use it with a local HTTP server to serve some Ubuntu PCs that have no access to the Internet at all, just to an air-gapped local network. When using the command line via "apt update" and "apt upgrade", everything works perfectly fine, I'm able to update, upgrade, install and remove packages like if I were connected directly to Canonical servers.

The problem is that the Ubuntu Software Center doesn't work in this setting. It only says that I'm disconnected from the Internet and it doesn't recognize the local HTTP repository that's configured and the rest of the system uses just fine. Users complain because they prefer to use the more intuitive GUI and that's understandable, but I don't know what else to do so I think this is a bug and no Google result seems to know how to fix this.

OS: Ubuntu 20.04.3 LTS right now, but the problem was present in past versions also.

What I expect to happen: the Ubuntu Software Center recognizes the local HTTP repositories and works.

What instead happens: the Ubuntu Software Center doesn't work because it says I'm disconnected from the Internet.