Consider this grid as an example. In grids, should I show the first page when user sort the grid by a column? I think logically I should. But I am not sure from UX point of view.
Given your example, there's no gain for the user if you stay on the same page. The reason is that there is no logical connection of the new sorting parameter and the current page.
A user is very likely to click back to the first page if they discover that the sorting did not reset the paging. So my suggestion is:
- Show the first page after resorting
- Be sure that it's clear for the user that they're back on page one (e.g. by temporarily highlighting the current page)