>> When app icon is selected (displayed on status bar), message notification appear "open" very briefly, then close, preventing the user the access to actions.
When app icon is not displayed, notification details stay visible
It's Android's automatically collapsing the notification and hiding action buttons. This is how it's always worked since Android 4.1 -- apparently they think that scrolling is bad or wrong somehow.
You can get the action buttons to show if you pinch-zoom-out on the notification. Yes, terrible UI, but that's not under my app's control in any way.