jekyll: Second configuration file is being ignored with --config _config.yml,_second_config.yml

What version of Jekyll are you using (jekyll -v)?

3.0.5

What operating system are you using?

OSX

What did you do?

I have two configuration files. _config.yml and _development.yml. _development.yml includes the configuration port: 1337.

I’m running the command jekyll serve --config _config.yml,_development.yml

What did you expect to see?

I expected the command jekyll serve --config _config.yml,_development.yml to serve the site from http://localhost:1337

What did you see instead?

The port configuration is ignored, and the site is served from the default http://localhost:4000

Notes

I noticed this after running bundle update github-pages. Previously I was on Jekyll 3.0.3 which worked as expected. On 3.0.5, the port configuration is still honored from yaml if it is in the first configuration file, but not in the second.