First of all, your expression is wrong on the screenshots. It must not be:
{{$_POST.IDMotore.default('null')}}
but
{{$_POST.IDMotore.default(null)}}
no quotes around null
I just tested this with the correct expression in an update record step and it works perfectly fine. Try correcting your expression and test again. If the expression disappears, try recreating the whole update step or the whole server action.