Last tested: Jun 12, 2020
This can happen when you create an alert on a field in a pivot and the value becomes null - since there isn't a value for the pivot column, Looker treats it as if it never existed. The pivot columns are purely based on data and not defined in LookML or the DB itself, so if it's not in the results, we don't know about it.
We can fix this issue by not limiting the data to one row, and instead showing a range, so that the pivot data are included. This will allow the alert to still function.