The latest iOS 26.4.2 update appears to fix the Signal notification bug.
… It looks like the iOS 26.4.2 security update addresses this, as Apple says it has fixed an issue where “notifications marked for deletion could be unexpectedly retained on the device.” Link: About the security content of iOS 26.4.2 and iPadOS 26.4.2 | https://support.apple.com/en-us/127002 | Apple …