The former couple hit up Madison Square Garden and both were onstage for Yams Day to honor the late music exec and rapper A$AP Yams ... who died 2 years ago Wednesday.
Kendrick Lamar, A$AP Rocky, French Montana, and Kendall Jenner were also there -- and this is really a case of squads colliding. Bella and Kendall are pals with Rocky ... so they were most likely there to support him.
No word if Weeknd and Bella ever came face-to-face, but it seems inevitable.
At least Selena wasn't there.