The service may disappear if there has not been any traffic to it within the selected time frame.
In order to see services with intermittent traffic, you may need to select a larger window from the time frame selector.
Process group must be there always even if there is no traffic to the services. Is the process really running on the host?
Services may disappear if there are not request for them, but process groups do not disappear.
But that means the process is not running on the host. Did you check the process presence? (you can check it by downloading the support archive in Dynatrace UI for the host, available in the 3-dot menu)
By the way - this is perfectly normal situation for IIS App pools in default situation - the process really stops and is started after next request to IIS.