First of all I really appreciate the addon for classic! There's nothing else out there that cleans up the clustermuck of addon buttons and looks a nice clean set of colors to go with my elvui scheme. That being said it stays open every reload and every log in for my character. I did go into the .lua myself and set the following:

["hidecompartment"] = true,

I just wonder why this isn't default? I don't mean to pick it, apart but when I need one of my 23 addons (ugh probably too many!) I can just click the master button then. Am I missing a command? Thank you again for the addon; that is simple and works smoothly.

The "hidecompartment" option only exists on Retail because the compartment frame of the default UI only exists on Retail.
Currently, there is no option that automatically closes the button on login/reload, it always restores the last state before logging out/reloading. But maybe the autohide option is something for you which automatically closes the button after a few seconds when you open it. You can set a time in seconds using this command:
/mbb set autohide <time>

Just to double check: Would you want the button to close itself on every login/reload or is it a different behaviour you'd like?