DLookUp returns the value of the specified column for one row matching the criteria (if any). Nothing garantees that the same row will be selected if you call the function twice successively with a different criteria, except if the criteria concerns the primary key column or a column with a UNIQUE constraint.
In your case, you should combine both conditions into a single criteria and call DLookUp only once (provided that the combination of both conditions constitutes a unique condition for the table):
Dim strCriteria As String
strCriteria = "field_1 = '" & Me.cmbo_scenario & "' AND field_2 = 'Accounts Receivable'"
If Not IsNull(DLookup("[field_1]", "tbl_detail_all", strCriteria)) Then