generally, i use my digitakt slaved to ableton via usb. when i do, digitakt will only loop the first 16 steps of a pattern, no matter what change in the settings i make.
when used as a master without ableton, it loops perfectly and patterns can have as many steps as I want to.
Do you have any clips sending MIDI to the Digitakt that could be restarting the pattern? Does it happen when you run the Digitakt synced to an empty project in Ableton Live?