The White House is desperately trying to crack down on leaks: https://www.nytimes.com/reuters/2017/03/03/us/politics/03reuters-usa-trump-secrecy-exclusive.html

The White House is desperately trying to crack down on leaks: https://www.nytimes.com/reuters/2017/03/03/us/politics/03reuters-usa-trump-secrecy-exclusive.html https://www.nytimes.com/reuters/2017/03/03/us/politics/03reu..., to the point of demanding aides hand over their phone to check that Signal is not installed. It's entirely possible the Post and/or whoever leaked the memo is suspicious of custom "fingerprinted" versions that could be traced back to the source.