I just tested firmware and default modes updates using iOS 26.2RC. I also successfully uploaded a custom command.
When iOS 26.2 gets officially released (probably next week), update your iPhone and then try again with the Maestro. Hope it works for you.
If that still doesn’t work, toggle the iPhone Settings - Security and Privacy - Bluetooth - Singular Sound MIDI Maestro off and then back on. Power cycle the Maestro and try to update the firmware again. When done, power cycle again and update the Default Modes. When done your phone may go to sleep but the Maestro blue pairing light might remain solid; if so, power cycle the Maestro and when it comes back on, open your phone and the app should display 100% Finished.
Let us know how it turns out for you.