Power Query is case sensitive, while DAX and the model are case insensitive by default. That mismatch can cause weird relationship key issues. Best fix: Avoid text columns as relationship keys. If you have to use them, normalize them in Power Query with Text.Lower or Text.Upper, or de-dup with a case-insensitive comparer like:  Table.Distinct(Source, {"letter"}, Comparer.OrdinalIgnoreCase).