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...
LaurentCouartou,
I got response from PG:
"This is a known product defect that's extremely risky to fix. We have communicated this to Alberto who initially reported this issue. We also communicated to Alberto that this usage of Summarize is highly discouraged and should be replaced by alternatives using AddColumns or SummarizeColumns. Considering the high risk of a fix and low benefit of a deprecated usage pattern, we decided not to fix the issue unless there is a strong business justification for it."
Reagrds,
Lydia