Mantis works via sessions, not option propagating within form posts. If you have navigated somewhere else, e.g. in another tab, it will use the settings for that. I just tested and it was okay so long as I worked on this basis.
Okay, still getting the same result and I'm not navigating anywhere else. I mean the Composr main website is still open in another tab but the actual link is missing the filter when I click page 2 without navigating anywhere else. Page 1 has it every time.
(Click to enlarge)
(Click to enlarge)
After fixing I saw it is this issue https://www.mantisbt.org/bugs/view.php?id=11569