Firebase Message not picked up by onMessage

[mobileappdev]

Hello.
We have create a PWA that used firebase-messaging-sw.js with onBackgroundMessage().
It also has onMessage() in a index.js file.
We programmatically send  a message from firebase that is just a message (no notification).
Inside the onMessage and onBackgroundMessage, we receive the firebase message and create a notification that is then displayed.
We are noticing that the backgroundMessage() seems to be accessed for some users when the app is foregrounded and the onMessage is then never accessed. Also the devices that do access onMessage correctly, do not then show the notification correctly. This is predominantly on iOS devices.
Let me know what might be the issue here.
Thank you