It's application based on rocketchat. This chat uses websocket for connecting and sending messages. I tried different devices, for example Nexus 5, Android 6.0.1; Samsung Galaxy S5, Android 6.0.1; Nexus 5X, Android 8.0. Actually, I have the same problem with iOS (iPhone 7, iOS 11; iPhone X, iOS 11; iPhone 5, iOS 10.3.3).
It seems to me, that I didn't set up some settings in Fiddler.