The lint filter in the door catches most of the fluff, not all of it. The finest fibres ride the hot air out through the flexible hose and the duct, and they settle wherever the air slows down: at every bend, at joins, in sagging sections of hose and behind the outside flap. Over months that becomes a felt lining, then a plug.
Lint is dry, light and catches easily, and a blocked vent traps heat around the dryer’s heating element because the hot air has nowhere to go. If your dryer keeps cutting out mid-cycle, that is usually its overheat protection, and a warning rather than a fault to reset and ignore. Until the vent has been cleared, do not run the dryer overnight or while nobody is home.