Sources connected to the now exes say they haven't been together publicly since his "Look Mom, I Can Fly" documentary premiered in Santa Monica on Aug. 27. They were there as a family ... with baby Stormi, but since then things have cooled.

We're told they've been trying to make the relationship work for a while, but several weeks ago they decided to step away -- at least for now. Worth noting ... Kylie hasn't posted anything with Travis since Sept. 10.

They were inseparable over the summer -- taking that crazy yacht trip with friends and family for Kylie's birthday. They also took a vacay together earlier this year -- in March -- after a rough patch where she thought Trav was cheating. He denied that.

He was noticeably absent this weekend in South Carolina, where Kylie attended Justin and Hailey Bieber's wedding ... with Stormi.

Kylie seemed to enjoy herself nonetheless, partying with friends like Jaden Smith. BTW, we're told there's nothing to that -- they're just friends and have been for years.

Kylie and Travis first hooked up -- at least publicly -- during Coachella in April 2017. She got pregnant within a couple of months ... giving birth to Stormi in February 2018.

Our sources say this isn't the first time they've taken a break -- and in the past, they've managed to work it out. We're told they'll continue to co-parent, and will keep Stormi as their number 1 priority.

For now, their new relationship status is ... complicated, but single.