The Founder's Cape cannot be applied to custom skins (the standard ones, not the new modular ones). This has been working in 1.14 beta.
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Comments 2
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-53513.
Please add a vote and any new information to the main ticket, which will be updated accordingly.
If you have not, you might like to make use of the search feature before making a new ticket, as it's likely that the issue has been reported already.
Quick Links:📓 Issue Guidelines – 💬 Mojang Support – 📧 Suggestions – 📖 Minecraft Wiki
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-53513.
Please add a vote and any new information to the main ticket, which will be updated accordingly.
If you have not, you might like to make use of the search feature before making a new ticket, as it's likely that the issue has been reported already.
Quick Links:📓 Issue Guidelines – 💬 Mojang Support – 📧 Suggestions – 📖 Minecraft Wiki