This iOS notifications flaw is known and goes also for other apps, like TG or whatever you want private.

Settings → Notifications → Show Previews → Never

Then you can you can allow individual apps if you regard them safe.

And in Signal app itself:

Signal → Settings → Notifications → Show → set to "No Name or Content"

This has been patched by Apple. Just download the latest iOS https://firefly.social/post/x/2047070518776356996?sid=1506745023