Software can’t run unless connected to live data??

I recently joined a new role where I've been tasked with troubleshooting a malfunctioning application built by the existing IT team. Upon reviewing the code, I noticed it lacked comments, unit tests, and documentation. Moreover, it requires a constant connection to our company's data systems, such as Elasticsearch and various databases, to function.

From my experience, it's generally advisable to design software where the front-end and back-end operate independently, without a dependency on live data. Is my understanding flawed, or is this situation truly irregular?

Additionally, I'd appreciate any recommendations for reading materials that emphasize the best practice of developing applications capable of running without live data. I wanna be able to reference them.