I am not sure what the exact reason is but I suspect that the orbit of the ISS with the Deltaglider docked simply degrades under time acceleration. Because the Re-entry processor currently don't calculate the time of re-entry burn (it probably should) and the correct time is very crucial to an exact landing, there is no other work-around than to load the scenario "DG_Reentry" which misses Baikonur by "only" 200 km. Re-entry and landing are certainly processes that need improvement.