That’s not true. The U.S. military has referred to the M-16 family, etc., as “assault rifles” for many decades.
I have never seen a DOD proposal for a new ‘assault rifle’
I have seen DOD proposals for a new Squad Automatic Weapons and Crew served machineguns.
Never have I seen a proposal for an assault rifle.
I suspect what you have seen is news articles talking about military weapons as assault rifles.