It’s definitely about the background image.
I see you have some CSS applied
I suggest that you create another 2 background images one for tablets and another for mobile
Alternatively, you could exchange the background image, with a background color only in responsive mode.
to achieve that, please use the following CSS, make sure it’s after the ones you’ve added
width: auto !important;
background: rgb(21, 103, 165);
position: initial !important;
Please let me know if you need my help