I am trying to do something similar and am having the same issue. I suspect that the issue relates to the javascript and jQuery that should be replacing either box with a search & select (jQuery select2) field. I noticed that a post_object field’s select box gets the class “select2-hidden-accessible” and an extra span with classes “select2” and “select2-container”. My custom object multi-select does not get that class nor the additional span element.
As for the “number box”, that is just what a multiple select field looks like when it is a single line. The arrows allow one to scroll through the options to select/deselect them.
