Tested the transparency of the Png format inside Moi :)
That works fine!
File Test is an object (yellow) mixed with 3 images png "transparent"


Some "plan" of image seems opaque for other following rotation on the axe "two by two"

You rotate all the view and the opaque plans became normaly transparent and other became opaque :)
Images Png used are "realy" transparent like helix4 (make your try)


Ps
Seems you have coll managed the names of files when the "same image" is loaded in the 3 views and Hide or Remove after:)