Table Showing last ~50 rows after filter removed

Hello Community,
I support a charity website which has a table connected to a dataset that has over 270 rows in it. A search feature enables either free text filtering on a text string or selecting from names looked up in a list (from a drop down list). The filter works perfectly well in either case, but as soon as the filter is removed the result set table in the page only shows exactly the most recent 48 rows. If you F5 and refresh the page, all 270+ rows appear again.
Can anyone suggest how I can figure out what is happening an why?!
Thanks ever so for sharing your wisdom.
Andy