We need to mark the query getting the affected number of rows as returning a result set that contains a row with the number of affected rows from the previous operation. Without it, the result would be read from the row count of the current SELECT

src/EFCore.Jet/Update/Internal/JetUpdateSqlGenerator.cs

@@ -116,7 +116,7 @@ namespace EntityFrameworkCore.Jet.Update.Internal
                 .Append(SqlGenerationHelper.StatementTerminator).AppendLine()
                 .AppendLine();
 
-            return ResultSetMapping.LastInResultSet;
+            return ResultSetMapping.LastInResultSet | ResultSetMapping.ResultSetWithRowsAffectedOnly;
         }
 
         //If multiple columns were output, the SQL Server behavior is to produce a INSERT INTO ... OUTPUT statement