Hello. If this is the case, it's because of your setup. The main configuration has two onscreen thanks messages... one is for Pay Later registrations and the other is used for everything else. Also, these two messages can be customized per event. If you enable the custom version of the message in the event, that one will be used instead.
So at the end of registration, if you see no message, this is because the correct message to display has simply not been configured. It's trying to display, but you haven't given it anything to show.