Is this a feature or a bug?
I have a fresh install of EE 3.5.15 (and have noticed this on other installs, thus my test on the fresh version). If I'm in the CMS on a page with checkboxes, such as the add-on manager. I click one checkbox in the middle of the row and several above it are also activated. It's not all above and it's not the same # of boxes above each time. And after you uncheck everything you are then able to check just a single box if it happened to be included in the initial auto selection.