On February 18th, Katie Arrington was named the Deputy Chief Information Officer for Cybersecurity and Chief Information Security Officer at the Department of Defense by the Trump Administration. She had served in the previous Administration in a similar role.
According to my source inside the Pentagon, shortly after Arringtons arrival at DoD she issued a waiver and authorized the various service CIOs to deploy Signal on government devices.
https://fpwellman.substack.com/p/exclusive-dod-has-deployed-signal
Apparently this came as a result of some of them throwing tantrums at not being able to take their personal cell phones (presumably with Signal) into SCIFs. The compromise was that they would leave their personal cell phones outside as long as they had Signal installed on their government phones.
This came in the same time frame as the same entity was issuing a warning about Signal vulnerabilities - including directing that Signal is NOT approved for processing or storing nonpublic unclassified information, and that all uses of Signal must abide by DoD and NSA/CSS policies.
So, while Signal isn't inherently evil, installing it on government phones is prohibited (absent a waiver). And while they weren't ordered to do top secret business on them - the app was installed on phones specifically so the phones could be taken into SCIFFs, where the person authorizing the waiver knew op secret information was being discussed/displayed - so that they could communicate via Signal with either with each other within the SCIF without leaving a trail - or with people outside the SCIF.