CSS Cache - Rebuild

I don't know if this is the correct place to ask this but I have the following situation:

I've created a custom sub theme based on the foundation base theme on a local test server.
I uploaded the custom sub theme to the web server and configured it for use.
Checked the site online and everything seemed in order.

Now I made some small changes on a CSS file and tested it on the local test server.
Everything was fine so I decided to upload & replace the corresponding CSS file on the web server.
Then I rebuilt the cache, checked the website on the web server and no changes were present.

I've done the following to try and solve it myself:
- disabled CSS aggregation
- delete the cache_bootstrap record where cid = 'system_list'
- delete sites/default/files/config_[hash]
- delete sites/default/files/php
- cache rebuild

I then used firebug to check which file was being loaded and I came across: mysubtheme/css/mysite.css?02l47t (the cached version)

Now to be sure that I wasn't loading an older version by accident, I:
- cleared local cache
- cleared memory cache
- bypassed & disabled the web cache (on my firewall)

Rebuilt the cache, reloaded the page and faced the same issue.
Is this a bug I'm encountering? Am I missing something?
Thanks in advance for any help.

Version information:
Drupal version: 8.0.3
Foundation Theme: 8.x-5.0-beta2