You can read more about this here https://dennyglee.com/2010/06/18/powerpivot-you-are-so-insensitive-case-that-is/
The article also contains some workarounds.
Can you use the EXACT function?
Measure 2 = EXACT("AAA","aaa")
Measure 2 = EXACT("AAA","AAA")
Proud to be a Datanaut!
Just to aid the conversation whomever is interested to reproduce the problem, try applying the following query
and check your results in either data model or the report view.
As per initial question from @googlogmob there is no such thing as collation in Power Query/Pivot that would be obvious to apply.
If the columns with Aaa and aaA are not meant for reporting I would suggest transforming their contents in Power Query to binary Text.ToBinary().