Created a new blank project in LCC Web with the single (default) layout. When clicking the screen resize buttons to switch between desktop, tablet and mobile things go a little awry. Initially the screen size for desktop is set to 800x500, tablet is 481x500 and phone is 480x500. Clicking back through the sizes (mobile > tablet > desktop) the desktop then appears as 976x500. If you then set your own size for the mobile option by keying it into the width field (I tried a width of 325) it initially sets it to 325, but navigating through the tablet and desktop sizes and then back to mobile you’ll find it’s been reset to 500x480.
Using LCC Web and Firefox 138.0.1 (64bit) web browser