I don't know if you've solved the issue as yet. This is something I've experienced in the past and never quite knew what it was; it happened only spontaneously for a short time and then would stop. It recently started happening to me again and it seemed to be because the AppCloudServices.StartAppCloudServices microflow was not running on start up.
I had a custom microflow configured in the Runtime tab of Project Settings (this is in Mendix 6.5.1) to run after startup, which was running the aforementioned microflow. However, for some reason, I guess it wasn't working. Try making sure the StartAppCloudServices microflow is running, if you have the AppCloudServices module in your project. Hope this helps.