Hey amirmeh, we came across this issue a while back and resolved it by making sure the language format for the survey and the end of survey message are the same. we found that our survey was in UK English and the saved end of survey message was in US English leading to the end of survey message to show the default message.
You can also change your preferred language option in your profile settings to prevent this from happening in the future and I recommend you cascade the message. If you create the survey and your profile language is in US English therefore by default the survey language will be in US English, then if someone else, such as a marketing rep, is creating the end of survey messaging and their profile is set to UK English then by default the end of survey message will be set to UK English and the miss-match happens without you realising.
in summary, make sure the language settings match for it to work :)
I hope this resolves your issue.

Thanks, Nick for your thorough answer. It all makes sense now.