The Founder's cape displays as a bib on the front of the player regardless of whether you are using a Character Creator skin, a custom skin, a marketplace skin or steve. This is consistent over realms and local multiplayer: when a player is wearing a cape, it is displayed for both parties as in front of the player.
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We're already tracking this issue at MCPE-54629, so I will resolve and link this ticket as a duplicate.
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