If the text is contained in a text-frame (TF) that started on a previous page, you might see the text on the first page inside the Web Editor. This is due to a limitation of the IDML format and the InDesign algorithm which allocates the text. This algorithm is not documented by Adobe, sadly. We suggest limiting the use of threaded text-frames across the document if you don’t explicitly need them (eg. in case you have long text overflowing).
InDesign ships with a pre-installed script called SplitStory.jsx which splits all the threaded text frames in just a few seconds. You can read more about it here.
Don't use threaded text-frames if not strictly necessary