Recently, one of my colleagues encountered the error "Field has not been explicitly selected" when they tried to update a field in one of the tables.
Usually, this happens when your X++ code or the runtime tries to access or modify a table field that was not included in the field list of a select statement or a data source query.
In this particular case, the root cause of the issue was the data source property "OnlyFetchActive".
They used a table extension to update the value of a new field in a table, but the field was not included in the select statement for the data source due to the "OnlyFetchActive" data source property.