I spoke with Heroku and I had totally overlooked the fact that they had bundler v2 available for Ruby 2.6.1, so I updated my build to use that instead and everything is building fine now.
I think there’s still a possibility that people will encounter this when utilizing 2.6.1 and bundler 1.15.2 and bundling gems for repositories, but hopefully most people are moving away from 1.15.2 at this point since it’s old and doesn’t seem stable with Ruby 2.6.1 at all.