It seems the issue is that you have added a space at the start of:
"sdk": "org.kde.Sdk",
so that it is instead:
"sdk": " org.kde.Sdk",
I even had to diff the manifest with a known good one to notice the issue.
It might be useful for flatpak-builder to check stuff like this, as i dont think runtime-id can have spaces in them in the first place.