It turns out that on some Android devices (such as Samsung and some Chinese manufacturer devices), when you run your app from within Android Studio, and you later terminate it by swiping it away from the recent apps, it will no longer receive notifications until reopened.
However, if you start your app directly from within the App Launcher on the device, and swipe it away, it will continue to receive notifications even after being swept away by the user, as long FCM Fallback Delivery has been implemented in your app.
This bug only surfaces when running your app from within Android Studio, and will not affect your end users' experience.