Not quite sure how this happens, but I see many sites with acf wysiwyg fields and they have images insterted into.
If user chooses other image size than the default(full) then it seems that wp will insert html code into img tag size=”1024″ or something that no css-rules can ovwerwrite. And this breaks the mobile views.
How should this be prevented ?
Can you use media queries for mobile screen
ACF runs they function in WP that makes images in content responsive the same as the WP content area. If an srcset is not added to the image then there are no other images of the same aspect ratio. The width shown in the attribute it the width that they user selected, if three is an srcset it is not necessarily the image that is being shown.
However, on a large screen this is the image being shown if the browser window is 1024 or larger and this is not affected by the container that the image is in. This is not an issue with what WP and ACF are doing but an issue with a user selecting a larger image than the container can contain. It is a misconception that images are responsive to the container, they are only responsive to the browser window width.