When trimming armor, it loads the texture successfully over the armor, but it goes over any other armor in your inventory, but only in your hotbar it seems. It doesn't look like it happens to armor that's already trimmed, just plain armor. It kinda follows the bug that happened with crossbows having incorrect textures of there were multiple and one was loaded while the other wasn't.
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We're tracking this issue in MCPE-168358, so this ticket is being resolved and linked as a duplicate.
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