In latest KDE Neon User edition, we cannot create flatpaks for arm, because of the qemu-user-static package. Installing the qemu-user-static package from Debian sid, arm flatpaks can be properly built.
I think you'll wanna take this up with ubuntu. We do not maintain the qemu packages.
Maybe we should? it's an important part of plasma mobile SDK. Or maybe you can help Dimitris reach out to them?
Unfortunately maintaining our own qemu build is very much outside the scope of neon. File a bug here https://bugs.launchpad.net/ubuntu/+source/qemu