After extensive testing I have found out that the problem is with the level on which the door is in relation to the surrounding. Each door needs to have at least 4 blocks in each direction on the same level as the door itself.
0000000000000
001111XXXXXX0
001111X1111X0
001111X1BB1X0
001111D1111X0
001111X1111X0
001111X1111X0
001111X1111X0
001111XXXXXX0
0000000000000
0,1 - ground levels, X - wall, B - bed, D - door