Smartphone lights up when another agent takes a chat.
Notifications for iOS seem to be bugged.
The phone (iPhone 7 and iPhone X) will light up without showing a notification when another agent takes a chat.
This does seem to be a bug related to the way tawk.to sends out iOS notifications.

Good day to you!
When visitor started a chat, we send :chat request: notification on apps.
If you from other device or other agent joins the chat, we remove “chat request” notification, since chat is already taken and we should not distract user. If such situation happened quickly, then of course you can notice that phone was awake.
If we would show “chat taken” it will be a mess in notifications bar, what no one will want.