We're told Ariana and Mac quietly cut off their romantic relationship after their work schedules became too busy, but they've remained best friends since then ... as always.

Our sources say the ex-couple love each other dearly and that will continue to be the case but, again ... just as close friends.

Ariana publicly announced she and Mac were an item in September 2016, and he was there for her during and after the tragedy of the Manchester bombing.

Less than a month ago ... Miller expressed his admiration for Grande on Twitter when she released her first new track since the bombing, "No Tears Left to Cry."

The song is from her upcoming album, "Sweetener," set to release in July.