Redirect https://example.com to https://www.example.com Heroku/Namecheap
The solution to this issue was to add both the root domain (example.com) and the www domain (www.example.com) to Heroku. Prior to this I had only a CNAME Record with a 'www' host pointed to Heroku. Now I also have an ALIAS Record with the root host '@' pointed to Heroku.
Thanks to @Ed. Kenbers for pointing me in the right direction!