I do tend to agree that in an deal world it should be possible to cross-build flatpaks, but: 1. You can't then test the output to make sure it even works. 2.

I do tend to agree that in an deal world it should be possible to cross-build flatpaks, but: 1. You can't then test the output to make sure it even works. 2. As a practical point, you could just do it in a VM