height of block with view containing exposed filter
Some html element wrapping the block has a fixed height. This height gets set the first time you load the page and thereafter it does not change (it could be done by a jQuery script). You will need to hunt it down in your web dev tools and alter it with a CSS rule that sets the height to auto, or you might even have to do it via jQuery.

Try with another theme (bartik) and see if it also happens. If it does not then it is theme related.

If you cant get it fixed then I suggest you create a custom region just below the content region that is used solely for the block. Just use a div for the custom region and only set the width if need be. On a block level element the height should by default be auto.

Look here how to add custom region.