The issue was that the Snowflake ADO.Net provider does not search for column names in a case insensitive manner (as the SQL Server ADO.Net provider does).
To fix the issue we had to surround the column names in the query with double-quote characters so that Designer could find the columns:
i.e. SELECT CODE as "Code",SUMMARYCODE as "SummaryCode", SUMMARYDESC AS "SummaryDescription""
This resolved the issue. The issue will be corrected within in the Q3 2024 version of Designer but would currently affect versions prior to this.