Many places online have pointed out to configure chromium proxy via command can be achieve with the following line

chromium-browser --proxy-server=[username]:[password]@[host]:[port]

but I got this result on every request.

Here's the output in the command line right after executing the previous command. (They do not appear to be relevant. There are no new command line output when I try to visit a page)

libGL error: failed to authenticate magic 30
libGL error: failed to load driver: vboxvideo
ATTENTION: default value of option force_s3tc_enable overridden by environment. 
[29551:29551:0606/160459:ERROR:sandbox_linux.cc(268)] InitializeSandbox() called with multiple threads in process gpu-process

I have double checked that the proxy credential works with the foxyproxy chrome plugin.

What else can I try to figure this out?

[Edit]

Going to chrome://net-internals/#proxy and reading "Effective proxy settings"

• if I do chromium-browser with no flags, I get

  Use DIRECT connections.

  Source: GSETTINGS

• if chromium-browser --proxy-server=[host]:[port], I get a message box requesting to login, and under "Effective proxy settings":

  Proxy server: [host]:[port]

• if chromium-browser --proxy-server=[user]:[pass]@[host]:[port], "Effective proxy settings" shows:

  Use DIRECT connections