Should be suffcient for the test, since you are not using any props in your field; but if you add props to the demo field, they need to be registered in the php file, yes.
then I’m puzzled.
Are you using different versions? I updated Kirby to the main branch but without success.
Will try PHP 8.1 later. I will have to upload it to another server for that.
When I tested this, I added a value in the content file. You can automate this by setting a default value. I don’t know if there is a way to fake a value.