This bug has been discussed in
another thread, apparently, the problem is that Blood and Wine uses the fact
q504_nilfgaardians_arrive to determine whether Ciri is empress, however, this fact is removed in the last scene of the empress epilogue of the main game (q504_10_cs_ciri_goes_home.w2scene). Adding the fact with the
debug console enabler mod before the epilogue of Blood and Wine works around the bug. I guess it might have been missed during testing because the different endings were simulated with debug tools, rather than actually playing through the game with different choices (which is obviously much more time consuming).