We also have noticed the same issue. However, we also noticed there were some changes in google-services.json if we download it again. We tried to create the same app again using new google-services.json file and it worked perfectly. You can try your hands for this things.
As logically you might understand, the app is compiled, it was sending notifications previously, we do not control the apps, it suddenly stopped working. It is an issue arising through Firebase Panel itself, we have put our efforts in order to get it easy for all of our users. We're not asking for excuses, as it works fine if you try to create a new app with the same software.
Hope, you understand us. Assure you all of our best services and efforts to make it much more better.