If you already have armor equipped and right-click to equip another piece, it will replace the armor in the corresponding slot. For example, if you try to equip a netherite helmet while wearing a diamond helmet, the diamond helmet will overwrite the netherite one and the netherite helmet will just dissapear.
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Thank you for your report!
We're tracking this issue in MC-275755, so this ticket is being resolved and linked as a duplicate.
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Can confirm