The main purpose of 1Blocker's system-wide in-app filter (Privacy -> Block In-App Trackers) is to protect your privacy against in-app tracking.
While it may prevent some ads from loading along the way, not all of them can be blocked using this approach due to the technical limitations of iOS. For example, YouTube and Instagram in-app ads are impossible to block even with the in-app blocking tool enabled.
It can occasionally block some ads inside iOS apps, games, and mobile browsers, e.g., Chrome or Firefox, but only if they rely on trackers or attempt to connect to tracking-related domains.
A note on iOS 27:
We have seen the Connectivity Assist feature (iOS Settings > Wi-Fi) interfere with the way traffic is routed on the device, which might cause trouble for 1Blocker's in-app filter, as well as other VPN-based tools. So, it is worth checking this setting if you are experiencing issues with an app that used to work correctly, or if ads in that app used to be blocked, before you updated to iOS 27.
Deactivating Connectivity Assist is the only option available right now. Since this behavior is caused by the system, it is not something we can change from our side, but we hope Apple will optimize it in a future iOS update.
Since targeting specific in-app ads cannot be guaranteed, ad-blocking inside apps is a nice yet sporadic addition to 1Blocker, as the primary goal of its system-wide filter is to prevent in-app trackers from collecting your personal information.