My Founders cape has vanished

Did you actually own the Founder's Cape? If you did not own the Founder's Cape and were only able to use it due to MCPE-84673 then this is working as intended. Only players who downloaded the Founder's Cape skin pack during Minecon 2019 should have the option to use the cape.

Oh, I thought it was just something for purchasing the switch version since it didn't appear until I purchased and played that version. Sorry for troubling you😅