I think the VM has to be switched off, not just suspended for the arguments to be editable. There are also some flags that UTM insists on and won't let you remove, which actually seems kind of reasonable - QEMU is really low-level and it's possible to run it in modes where UTM won't be able to connect to it.
I wanted to run some risv emulation, and I need to remove some device from the default args UTM has, and nope. Not editable.
Unless I'm missing something..