I've found out the problem ... by adding a "rustc -V" command in the manifest's build-commands at the top, i can see which rust version the flatpak env is using.

When you use org.freedesktop.Platform v22.08, the rust version is 1.72.0. If you use v23.08, the rust version is 1.77.2. And if you use v24.08 (the latest), the rust version is 1.81.0.

So setting "runtime-version" to "24.08" in the flatpak manifest should fix the flatpak build problem !