When I add a leather armour piece to the armour stand, the armour stand shows a iron armour piece instead.

Recently leather armor pieces show up dyed white. This could be the case. Is the texture slightly leather-like? I do not know why they show up dyed, this might be a bug. Mobs also spawn with white dyed armor, but drop regular.

Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-44669.

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Bugs regarding the game Minecraft belong in the MCPE project. Moved to the correct section