Data type issue when applying a relationship

Hi PaulDBrown ,

Like everything else in Power BI, there are no absolutes, but a few things about key fields vs value fields:

1. Key fields should be unique on the 1 side. I'm ignoring the complexities of the many to many relationships.
2. Key fields should generally be hidden on the many side. I've seen a lot of problems in reports where the key field was used in a visual in a FACT table vs the DIM table. Doing this with dates will prevent time intelligence functions from working.
3. As noted by the OP, the sigma sign is vanishing, and while that appears on the face of it to be a visual issue, I don't know what SSAS is doing underneath when that field is used as a filter relationship.
4. While I am sure someone will come up with a scenario, what possible mathematical use would a unique value have in a table besides counting? If you are just counting, you can count the rows. But you shouldn't be dropping that field into a visual to create an implicit measure, not from the FACT or DIM table. You could drop it in a table as a column or matrix as column/row as you would a part number, but not in values. 
5. Unless it is a unique ID generated by SQL or other database, I almost always convert numeric IDs to text before using them with filter relationships. It ensures they don't break if any alpha chars get introduced. Part numbers, employee numbers, etc.

I would just recommend creating a duplicate column in Power Query, then using that. Your "many" key field should be hidden anyway, so wouldn't cause confusion.