What is the column type of the column which is selected in the consumer field when configuring connection between query string web part and list web part?
If the column type of the consumer column is the Checkboxes (allow multiple selections), when you filter multiple values on the column, it will only filter by the first parameter and skip remaining parameters.
The article below is for SharePoint server 2007. In SharePoint 2010 or 2013, there may be some changes, so the query string web part will not work as the article description.
There is a workaround.
If the column type of the column is the Checkboxes (allow multiple selections), you could copy the value from the column to a single line of text column.
Edit query string web part, in Advanced Filter Options, select “Combine them into a single value and delimit with , “ like the picture below.
Then configure connection on the sing line of text column between query string web part and list web part. In my environment, the ss column is the signle line of text.
Then use query string in the URL, the result like the below.