Custom member directory profile columns not responsive

I’ve installed the latest WP and UM versions

I made a custom template in my child theme /ultimate-member/templates with the <?php /* Template: Login 2 */ ?> tag, so my “Edit Member Directory” page picks up the new template fine.

Nothing has changed from the members-grid.php template (just copy and pasted that into my new template) – But, the new membership page has 3 columns instead of the 2 default columns and when the screen narrows the columns don’t fall into 2 then 1 column, instead the columns stay as 3 and just get really, really, narrow?