I extracted this issue from an 2016 post, as the "recent" update to the SUMMARIZE behavior did not fix the issue described there ( https://community.powerbi.com/t5/DAX-Commands-and-Tips/Is-there-sth...
I believe we should clearly state any limitation in power bi documentation as well which is missing currently on the main page .
https://docs.microsoft.com/en-us/dax/summarize-function-dax
I have prepared a short video on just the basic usage of summarize .Hope this enables users to use this function correctly.
Do watch the below video