When you validate an Arcade expression, Pro takes a feature (the first?) from the table, applies the expression to it, and returns errors it encounters. This validation does not take into account definition queries.
For example: I have a text field with date values that I wanto to convert into actual dates. In the layer, I have a definition query that filters out null values. The Arcade validation doesn't honor that query and validates the expression with an empty value, giving an error.
The actual tool does honor the definition query, leading to the absurd situation that the tool runs successfully although the expression failed to validate. Also, it makes debugging really difficult, because there's nothing wrong with the expressions or the (visible) values in the table.
Please make the Arcade validation honor definition queries.