I'm in a birthday party planning group on Signal, which is another app I hadn't used in years. It's easy to forget I have messages there because it's not on my home screen, or the notification settings are different from my normal Messages app, or I just forget to look.
Each time you use a "different" app for something that's not in your habit loop, your response time gets delayed and you're less likely to notice communications. An app needs to get fairly regular use to become as useful as a primary app, even if, from a technical standpoint, there's nothing wrong with them.
It seems like Apple has some difficulty to adapt to some international audience.
IOW: When I started using signal again, I didn't set aside 5 minutes to analyze the differences in notification settings between my messaging apps and consider the implications of them. I just opened the app and typed a message.