As reported at https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Bug_in_Special:Contributions_-_lost_checkboxes:
If I go to the contribs page for a given user - say Special:Contributions/Redrose64 - open up the "Search for contributions" collapsed box, and select a namespace, I am then offered two more checkboxes: "Invert selection" and "Associated namespace". Whatever I do with those two, if I click on Search, then use my browser's "back" feature and open up the "Search for contributions" collapsed box again, the chosen namespace is still selected, but the two checkboxes are no longer present. To get them back, I need to select a different namespace, then reselect the one that I wanted first time. Firefox 99.0.1, all skins. --Redrose64 🌹 (talk) 20:45, 3 May 2022 (UTC)