This is related to the combination of the column width settings and view port.
When opening the column properties, you can set the with per device/viewport. When you want to force a single column on the mobile viewport, set it to manual/12 for each column.
If it is a dedicated mobile page, (not responsive and used for desktop as well) then you could just add separated rows per item.
//Note/// You are using 8.7?? small advice, use 9.x instead