This issue appeared because you used your phone with your apple watch, and the problem is that you received the notification on your watch and not on your phone.
The notifications can be received only on one device (https://support.apple.com/en-us/HT204791), and the problem is that when you receive a notification on your watch then the preset sound of the notification is not displayed on the phone.
If you want to hear the sound of the notification when you use the app with an Apple Watch, you will need to turn off the notification mirroring in Watch app on your iPhone, but in this case you will not receive the notifications on your Apple Watch. Unfortunately this is the current way Apple integrates the iPhone and the Apple Watch. There is no workaround.
