Zombie Pigmen are the only mob that spawn on wither roses

I am playing on ps4 and in a realm on the latest update.

I took out a nether fortress except a 40x40 area of a cross road within a nether fortress. The cross roads spawned multiple mobs including wither skeletons. Then I replaced the top level of blocks with netherrack and covered them with wither roses. Then only Zombie pigmen spawned (and died). I replaced the nether rack with soul soil (and wither roses) and got the same result. I then encased the entire area with tinted glass (9 blocks high) and still only zombie pigmen spawned (and died). I took out the soul soil (and wither roses) leaving the cross roads one block shallower than it was originally generated and it produced mobs again.

I then went into a single player world and did the same tests and it yielded the same results.