Once again: the applications remain open, they just don't appear on that screen. There is no workaround that I know of, but then again you don't want to be scrolling down forever, right?
Also if youi long press on the screen so you can close selected apps you get a chance to view more than 9 (not sure what the limit is though). Also bear in mind that if you open one of the minimised apps and Jolla puts it to the top of the multitasks screen. So the ones that have slipped out of view are those you have probably not been using.