As I understand it, the Admin SDK Code has supported the HTTP v1 API for quite a while, but you have to use the messaging().send() Method instead of sendToDevice() (and other sendTo*-Methods), since the deprecatred sendTo*() Methods use the Legacy URLs. Someone please correct me if I'm wrong.

thanks for the complete response @mikejpeters i will try it now

We recently migrated our push notifications from the Legacy HTTP API to the HTTP v1 API. Since then, push notifications are not working on either iOS or Android devices. Although FCM returns a "success" response, the messages are not being received by the devices.

Here's what we have done so far:

Migrated to the HTTP v1 API following the Firebase documentation.
Verified the entire code, especially the admin.messaging() module from the Firebase Admin SDK (we're using the send() method).
No errors or outages reported by FCM.
All relevant SDKs are up to date.
Tested whether low priority or message lifespan could be causing the issue – no success.
User devices are not in sleep mode and the app does not crash upon receiving the notification.

Has anyone experienced a similar issue or have any ideas about what could be causing this?