“Track” means that you can only set the same loop length for all parameters of a track, i.e. a track will have the same loop length for triggers, notes, octaves and so on. To select a different loop length for a track, you either have to be on the first page (triggers) where you can see all four tracks at the same time or select the correct track with the four left-most buttons on the bottom row.
Try this: Go to the first page, press and hold the first of the three modifier-keys in the bottom row and then set a loop for one track by pressing and holding the first step of a track and then pressing a different step on that track - the loop should only change for that one track and it should be the same length for all the parameter-pages of that track.