I've tested this on my friends HTC one X and he has the same problem.
Are you guys experiencing this problem too?
I've got the led notification for group chats turned off and the led notification for the normal chats on. When someone talks to me when my screen is off, the led starts to blink. But when someone starts to talk in a group chat, the blinking suddenly stops. So i miss notifications on 1 to 1 conversations when group members are chatting actively.