Within (Patreon)

As suggested by Singarti

As a human and orc merge, the orcs soul slowly seeps into the human., changing it on a metaphysical level. This is why when they unmerge, a human may take on orcish characteristics, that slowly fade with time out of an orc body. However, if a human spends enough time merged, they may begin to fully become an orc even after unmerging. The rate of which depends on how synched their souls are