When establishes the two-phase interface of DPD in lammps, due to the use of periodic boundary conditions, the material of the upper phase will go to the bottom of the box. Does the Pxx, Pyy and Pzz of the thermo_style output system have an impact?
What you are seeing is your system drifting. No particles get “transferred”. You have to look at this with the periodic replica of your system added to understand this better.