when using a multiple select list, the selections are sent as an array.
the array needs to be converted to a comma separated list.
In the data assist search behavior, you also need to set the criteria that is using the select list to a Keyword criteria type, and set the OR separator to use a comma followed by a space:
", "
send a copy of the search page please and I can give you code you will need to convert the selections from an array to a comma separated list.