Thank you for reporting this important issue.
I have recreated this issue in a test environment. I’ve send Jason (Lead Developer) a full report of this bug. In the meantime, I can give you a hack to get this working in the short term. I can confirm that translating these phrases does work when you use the methods provided here:
Knowledge Base » Changing Words/Phrases In s2Member®
You’d want to do something like this within your Must-Use plugin, in conjunction with using your .mo file.
Make sure you put your .mo file into the main Plugins Directory, instead of leaving it in /s2member/includes/translations. If you leave the file there, it’ll get overwritten when s2Member updates.