Email newsletter not rendered properly

problem
#1

I subscribe to an email newsletter, Level Up with Ruby. The email, when rendered through Apple Mail (mail.app) on macOS Monterey (12.2.1) looks good. When I view it on the Newsblur site, the code examples are not displayed. When I view through my RSS client, Reeder, the code examples are not displayed. See screenshots

Level Up with Ruby #3 Safari at Newsblur
Level Up with Ruby #3 Safari at Newsblur1357×933 41.7 KB
Level Up with Ruby #3 mail.app
Level Up with Ruby #3 mail.app1296×975 67.5 KB
Level Up with Ruby #3 Reeder Mac
Level Up with Ruby #3 Reeder Mac1422×965 63.1 KB

I have created a [gist with the source from the email] (Level Up Email Source · GitHub) - nb email address replaced for privacy purposes

I am not sure if this is a problem with ConvertKit or with Newsblur (or likely some combination of the two), or a problem with my setup.

Thanks in advance for any and all help.