Thanks for bringing it up in the call, Jeremy.
I was able to fix this problem. The old value was stored in the sprockets cache directory in a .cache file (under tmp/cache/assets/sprockets). [thanks grep!] Deleting that directory and running assets:clobber then assets:precompile and restarting puma finally pushed the new value into the application js file.
I'm not a Rails dev, but I think this problem happened because Rails didn't know to regenerate Hyrax's app/assets/javascripts/hyrax/config.js.erb file using the new config values because that template hasn't actually changed. It would fall back to using the cached version instead.