Allow role to add a new record but not modify existing records on a connected table

I think in the end, for this particular problem, you will need to choose between allowing Grid Edit and controlling editing on that single field. So one solution is to disable Grid Edit on that form and then when your Users go to edit they will have to Edit on the form where the Dynamic form rules will apply.

Dynamic form rules are called dynamic form rules because they only apply dynamically on forms, and not in Grid Edit mode