Some creatures can't open doors
In Morrowind.exe, there are two flags that, when set, allow a creature to open a door: the Bipedal flag, and the "Weapon & Shield" flag. In OpenMW, only the Bipedal flag is recognized for allowing creatures to open doors.
In addition, in Morrowind.exe, a creature will automatically cause a door to open simply by colliding with it, including having a door closed on them (preventing them from being trapped). In OpenMW, a creature may only open a door if it has fully closed, allowing players to trap creatures within a doorway.
You can test this by spawning any non-bipedal, weapon using creature (such as a riekling, bm_riekling) behind a non-load door, preferably in an interior.