Hi, Dan
For I cannot reproduce your issue, I have some questions and suggestions:
When you say "suddenly changes" do you mean in the Designer's Preview or after you receive the email?
There are several different clients be able to selected in the Inbox Preview tab.

Please check the effect of Outlook 2013 here to find out if there is a problem with the preview effect. According to my tests, the effect in Preview does not show font changes.

Please also check that creating such Bullets directly in Outlook 2013 does not cause the font to change. If yes, you’d better turn to Outlook for help.
Looking forward to your reply.
Best Regards,
Nya