This is happening because the main div has
overflow: hidden on it, so it is hiding the part of the select menu that extends beyond its borders.
The solution is to remove
In certain cases, the overflow setting is necessary for the design and other things might break if you change it. This is unfortunate, and I know of no good way to get the Materialize select to work with this. In my app, I modified the select so that the list of options gets appended to the document body, which means it is always above any other content. This is how, for example, the Angular Material select is implemented.