After a few tests the answer is from
https://learn.microsoft.com/en-us/sql/analysis-services/tabular-models/string-storage-and-collation-in-tabular-models?view=sql-server-2017
In short, e.g. if in the source table we have product name like 'bike', 'BIKE', 'biKE', tabular model will save only the first value. When processing the model, SSAS will evaluate the string value only. Now if we just want to update 'biKE' to 'BIKE', unfortunately because of existence of 'bike', the report still shows product name as 'bike'.
This is a bit interesting. Because when we build data model, we usually denormalise parent-child relationships so obviously we could have duplicated entries. If we have product departments want to display product name in different cases, e.g. Asia department wants to report product name in all upper case, AU department wants to report product name in proper case, we have to build an extra format string.