The welcome message, that appears under the photo of affiliate manager in the affiliate panel, needs to be enclosed between ## and ##.
So when editing the affiliate manager info (welcome message) in the merchant panel by clicking your merchant name > Change affiliate manager info visible in affiliate panel, then it is supposed to look like this:
##Hello and welcome to our affiliate program...##
If you changed the welcome message, ten you need to add it as a new translation into your translations (in merchant panel navigate to Configuration > Languages and Regional settings > Languages tab > Edit language (in actions column) > Translations tab > Add custom translation)
NOTE: There is also a Home screen welcome message