During the season 6 reunion episode, which aired on Peacock Monday, Nicole and Kendall confirmed they were in a tough spot after linking up on the dating competition ... with NJ accusing KW of lying to her face about his viral nude leak.

Here's the skinny ... NSFW footage of Kendall dropped online as he filmed the reality show this summer, with the TV personality later claiming the leak came from someone from his past.

However, Kendall admitted during the reunion that he lied about when the video was taken ... as it was actually filmed in a hotel room shortly before he joined "Love Island USA."

And the recipient of the footage wasn't a trusted ex ... rather, Kendall had sent this clip to someone he had met on a dating app.

While this all took place BEFORE Nicole and Kendall got together in the villa, Nicole was still distraught by her beau's dishonesty ... saying she learned the truth from fellow Islander JaNa Craig.

She added ... "I think if you say you love somebody … you need to be honest. Instead of saying [the video] is from years ago."

As for Kendall, he defended himself, saying he was just trying to put his life back together after it was "turned upside down" by the leak.

Tension between Nicole and Kendall was palpable, and both felt the other pushed away in the aftermath of the drama.