What does the subject line look like inside your email editor?
This is so strange, I cannot reproduce your issue. It leads me to believe it may actually be the recipient email client not rendering the subject correctly. If you send a sample to a different account with different email client, does it work? I have tested using the following subject and it works for me on Outlook 2016 for Mac as well as iOS Mail and Outlook for iOS:
Vous êtes invité(e)... Soirée cocktail d’appréciation de nos clients VIP – 16 novembre, Ritz-Carlton, Toronto
For some reason this also happens when sending to other email recipients. I've been researching and sometimes these odd characters can occur with subjects that have 75+ characters. I've tried to reduce the number of characters to 56 with spaces and everything looks normal right now!
Thank you for your help on this