Non-empty manifest read as empty

[ThePeach]

This is an odd one. On my local machine everything works as expected. I deploy the application on an ECS instance and when I ty to load the login page I get the error:

Processing by SessionsController#new as HTML
  Rendered sessions/new.slim (Duration: 0.6ms | Allocations: 384)
Completed 500 Internal Server Error in 5ms (ActiveRecord: 1.0ms | Elasticsearch: 0.0ms | Allocations: 1009)

ActionView::Template::Error (Shakapacker can't find admin.css in /app/public/packs/manifest.json.
[snip]

Your manifest contains:
{
}
...
[snip]
     8:     = stylesheet_pack_tag 'admin'
[snip]
app/views/sessions/new.slim:8
[snip]

when it's clear that the manifest in public/packs/manifest.json contains more than a few things:

# grep admin.css public/packs/manifest.json
  "admin.css": "/packs/css/admin.css",
  "css/admin.css.map": "/packs/css/admin.css.map",
          "/packs/css/admin.css"

I am at loss, where is it reading the empty manifest from??? 🤷