Hi!
I am afraid this cannot be turned off now. The Isotope rows count can change during the orientation change (due to different screen widths), and that may result in different item numbers per page, and different number of pages. There are lots of other issues with it as well, so the script simply resets the isotope handler, and reverts it to the first page automatically.
I have noted this, as there were an update to some of the 3rd party scripts (including the isotope script), so I will investigate this before the upcoming release. Maybe there is a way now to bypass this ‘reset’ or only trigger it when it is absolutely neccessary.