I don't know precisely when, but in Ubuntu_xenial_next_desktop.7z "chromium-browser --start-maximized" does not start the browser maximized.  In previous versions of armbian it works properly.  I tested chromium 61.0.3163.79 (same version that is on Ubuntu_xenial_next_desktop.7z) on my kde neon system and it works.

Sorry I can't duplicate this now.

I found how to duplicate it... sort of.  It seems that start-maximized works only have you have resized (or maybe moved) the chrome window.  Once you do that then start-maximized will work properly.

Ok I can reproduce it.  Fresh install...  run "chromium --start-maximized" and chromium starts up not maximized.  Resize the window, exit chromium and now run "chromium --start-maximized" and chromium will be maximized.  Moving the window has no effect, you must resize the window first.

Resizing the window looks like it writes something into .config/chromium/default/Preferences that isn't there initially.  This prevents start-maximized from working I presume.

All this said this is probably a chromium bug.