KWallet Secrets Portal can't get secret for Flatpak

I wanted to try out the XDG Desktop Portal secret functionality in Frameworks 6.2.0, so I tested it with Pika Backup from Flathub. However, attempts to save the encryption password fail, and eventually the following error message appears from KWallet:

There have been repeated failed attempts to gain access to a wallet. An application may be misbehaving.

I’m not sure if this is a bug or if there’s something I’m missing, so I thought I’d ask here first. I only have one wallet which, according to KWalletManager, is open when the error occurs. Is there anything else I should check for?