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Amanda Nunes is calling BS on Cris Cyborg ... telling TMZ Sports the former UFC champ NEVER would've left the UFC if she really wanted a rematch with the champ-champ.

Nunes -- the bantamweight and featherweight champion -- came by the office this week with her fiancee (and #5 ranked UFC strawweight) Nina Ansaroff ... and chopped it up about Cyborg.

Remember, Cyborg -- who was KO'd in the 1st round of her December 2018 fight with Nunes -- floated the idea of a UFC-Bellator cross-promotion fight in the future when we saw her at LAX last month.

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But, straight up, Nunes ain't buying it.

"Honestly, I love a challenge, but she made a wrong decision. She left [UFC], why? Why? If she really wanted to fight me she should’ve stayed."

Amanda says if Cyborg truly wanted the fight, she would've done anything -- including staying with the UFC -- to make the fight happen.

"If [Cyborg] really wanted [the fight], [she'd say] 'I’ll fight Amanda, give me $10,000 dollars. I'll fight her for $10,000. I’m better than her, I want that belt back.'”

So, Amanda thinks Cyborg's lying ... but it probably doesn't even matter.

The UFC hasn't done a cross-promotion in over 15 years ... and then there's the whole issue of Dana White despising Cyborg.