The test Doorbird I have had installed for a few months now has done this. Away from the local network (using 4G LTE or another network), there would be times the Doorbird would not connect to its app. This could last for a few days before it started working again.
But on the local network, the app would always connect fine (even when it was giving trouble outside the local network).
Also during these times of off-site connection issues, the Doorbird would still send images to the cloud storage. Which would indicate it is an app issue.
This test Doorbird is also connected to a local NVR, to record the stream. And that has never been interrupted either.