pbiplintRules / Error Prevention
Relationship columns should be of the same data type
What it checks
Relationships whose two columns have different data types.
Why it matters
The engine relates columns by value, and when the types differ it converts one side for every query. A text key on one side and a whole number on the other works until a value like 007 meets 7, at which point rows quietly fall into the blank member. Matching types remove both the conversion cost and the surprise.
How to fix it
Change both columns to the same type in Power Query, and prefer whole numbers for keys. In the TMDL file the property is dataType on each column.