Starting with macOS 15 Sequoia, Apple’s built-in firewall is causing troubles when used together with third-party firewalls that are based on Apple’s Network Extension framework (which is actually the only way for third-party developers to create such firewall products for the Mac).
As a consequence of this new bug it can happen that Little Snitch doesn’t receive any network traffic information from Apple’s framework, hence no traffic is then shown in Network Monitor, no connection alerts are shown, and firewall rules for blocking connections are not applied.
For the time being, until Apple fixes this serious bug in macOS, we therefore highly recommend to turn off the built-in firewall of macOS when also using Little Snitch or Little Snitch Mini.
It’s worth mentioning that the Little Snitch firewall also works for incoming connections, providing the same functionality as the macOS firewall (except for stealth mode), so it’s usually not necessary to have them both running simultaneously anyway.