Phone notifications are not working on VA3 when a Connect IQ app session in running

When running a 3rd-party app on the Vivoactive and Vivoactive 3 (maybe other devices are affected as well) no notifications are available during the time a workout session is running. 

The notifications are enabled in the watch settings and there is a valid BLE connection between phone and watch.

In my case the problem occurs with iOS.

The problem does not occur e.g. on the Fenix5x.

Would be great to fix this soon as this is an important feature on a smart watch.