Confirming the same issue on the Framework Laptop 13 DIY Edition AMD Ryzen AI 300 series / AI 7 350. After spending several hours trying various things I came across this post, then I decided to try to install Ubuntu 24.04 to do the firmware upgrade and that worked flawlessly. After the firmware upgrade I rebooted with the same 26.04 disk and it worked without any issues!
So yeah if you come across this problem, install 24.04 or the server version and make the firmware upgrade, then you should be good to install 26.04.