How to implement a select list that allows for multiple selections?
From the various attempts you tried, it seems that the reason for your issue is simply that you have not indicated that "multiple values are allow" for this field.

To ensure multiple values are allowed, navigate to /admin/structure/types/manage/mytype/fields/field_abc (assuming 'mytype' is the machine name of your content type, and 'abc' is the machine name of your field). As an alternative, use the "edit" link for your field, within the "manage fields" tab for your content type, after you selected the content type via Admin &gt; Structure &gt; Content Types.

Then within the option that says "Number of values", indicate the "Maximum number of values users can enter for this field.". Note: the default is 1, so if you did not change that yet (eg to 'unlimited'?), then that would explain your issue (and resolve it).