1. You need to change in style.css the portrait or max-width to account for the new space. To avoid the following:

2. You should give a little more padding to the new button, notice the difference:

3. It's preferable that you allocate the same width for both states of the button instead of changeable width, as the later generally provokes responsiveness issues. Also a general better text could be "Unassigned"/"All Issues", i.e: something shorter.

4. The mobile view is very broken. I think you could split the space between search bar and it in a 3/1 flex split, if that works. Stacking vertically either makes it very convoluted, confusing and also violates the bottom-bar max height (that could be changed, but I dislike the approach)

I'd create a div that wraps search box and the new button has display: flex, then in search box I'd do flex: 3, and in button/label flex: 1. Be careful of how it impacts the desktop view.