As much as it looks like appletonium (or wolterianum), bullenianunum comes from Borneo and has a beefier flower.
Cribb says that this complex is a mess with very similar plants:
P. ambile
P. robinsonii
P. linii
P. johorense
P. celebesense
P. tortipetalum
all species status names circulating freely in cultivation for what is probably the same plant.
With a comparison of the incredible amount of variation Averyanov has documented for appletonium in Vietnam, I wouldn't be surprised that bullenianum would have a ton of variation throughout the south pacific.