If ya missed it, Quincy Jones was first to blow the whistle on MJ ... saying he outright stole that famous "Billie Jean" bass line from Donna's song, "State of Independence."

We took Q at his word when we read the Vulture interview ... and listened to both songs. Once we slowed down 'Independence' it was blatantly obvious Quincy was right. Now, for the plot twist!

MJ was part of the all-star lineup that sang backup for Donna's song. Shout out to the loyal TMZ user (@Rascoundrel on Twitter) who pointed out this video ... showing Michael, Stevie Wonder, Dionne Warwick, Lionel Richie and Kenny Loggins in studio. And yeah, Quincy was there producing the whole thing.

Now, Donna and Michael were friends ... so, it's possible she was aware he was lifting the beat. We just don't know.

Of course, Michael still wrote original lyrics and added his signature "hee-hee" all over "Billie Jean" -- but c'mon ... case closed on the beat.