In most cases this issue is caused by elements, which overlap or are positioned outside the defined page/breakpoint width.
You definitely have to avoid, that elements overlap or are positioned outside your defined page/breakpoint width.
If there are such elements, browser normally shift horizontally of zoom in to make all elements visible.
To control this,
zoom out considerably, perform a „Select all“ (Cmd+A) and see, if there are such elements. Do this in every breakpoint on every page, including your master page.
set guidelines and frame borders visible and drag the scrubber (this grey, vertical handle top right of the breakpoint bar) slowly inwards to verify, if there are elements bleeding outside the breakpoint/page width like you can see in this example:
If you find something like that, correct it, by setting the elements to fluid width, making them smaller in width or correct pinning.
If none of these hints helps, delete all 3rd party widgets one by one and see, if the issue remains.
If this doesn’t help too, share a small(!!!) .muse file with only one page and some elements with us, to have a look at. Please follow these instructions: https://forums.adobe.com/docs/DOC-8652