Gem assets in middleman with middleman-sprockets

I’m trying to include stylesheets from a custom gem in to middleman through the @import. However it seems like middleman-sprockets isn’t finding the correct path? The path that it’s trying to hit is http://localhost:4567/stylesheets/ According to this should just work though. Is this functionality just deprecated in favor of using the external-pipeline and webpack?

I do know that this isn’t the gem, because I’m importing it the same way in a Rails app, and it is working as expected. This gem is from a private github repo, if that makes any difference.

I’m using:
middleman: 4.3.6
middleman-sprockets 4.1.0
ruby 2.6.5

Not an expert here, so this might be a longshot, but I imagine you’ll have to tell Sprockets explicitly to pick up your asset paths.

Perhaps similar to how it’s done in font-awesome-sass?