Azure Notification Hubs - Dropped Android FcmV1TemplateRegistration

Question

Using the this REST API to retrieve list of Android FcmV1 registered devices on Azure Notification Hub.

https://[my_namespace].servicebus.windows.net/[my_hub]/messages/?api-version=2015-01

Works perfectly, here is example of an FcmV1 registered device in the Notification Hub list;

i                   : http://www.w3.org/2001/XMLSchema-instance
xmlns               : http://schemas.microsoft.com/netservices/2010/10/servicebus/connect
ETag                : 1
ExpirationTime      : 2124-07-22T04:38:31.6331043Z
RegistrationId      : 4282634567123812146-7158421617157755944-2
Tags                : 1001011011011001
FcmV1RegistrationId : fdkyg1j9Tm6zlRrHQX3mGW:APA91bGU_...shortened...hpJK9txyzgjsz-IFyUIJ
BodyTemplate        : BodyTemplate
TemplateName        : TEAM

Devices self-register (more or less forever) and refresh using the Android SDK;

com.microsoft.azure:notification-hubs-android-sdk:2.0.0

Works perfectly, except... sometimes the registration disappears from the list.

Device won't get notifications if Notification Hub device registration not present.

When devices refresh their registration on App startup/open, they reappear in the list with the same ANH RegistrationId and FcmV1RegistrationId and get notifications again. The Registration Ids are persistent.

Normally, the App gets notifications regardless of whether it's in foreground, background or not even running.

ANH API happily continues to accept/send to the dropped registration. Of course, it gets nothing.

Don't think ANH tried to send the notification to Firebase. It can't without FcmV1RegistrationId, while no error is returned and process doesn't know it failed.

Previously, this behaviour was different with FB Gcm registration implementation, which threw an error if a valid GcmRegistrationId wasn't found.

Further, our direct Firebase HTTP v1 API (JWT) protocol successfully sends to devices that aren't registered on the Notification Hub, so the FcmV1RegistrationId is definitely valid.

Question: Anyone else having this problem and how can we be certain that FcmV1 registrations never get "dropped" by the Notification Hub..?