Eyewitnesses tell us ... Kelce's former flame -- who he dated for years before Taylor Swift -- and the Chiefs QB's younger bro came together at the Aria's High Limit Lounge around 10:15 PM Thursday, and they didn't leave each other's side for about an hour.

Sources with direct knowledge tell us this meetup wasn't planned -- it was more of a bump-into sitch, apparently -- although folks at the casino say it seemed coordinated, as we're told Kayla joined Jackson at the hotel's fancy bar after he'd been waiting around for a minute.

So, to some, it appeared Jackson might've been waiting for her.

Regardless, witnesses say Jackson and Kayla -- who became good friends during the years the model dated Kelce -- were warm toward each other while they talked, appearing jovial throughout their entire conversation ... which, we're told, was an engrossing one for both.

Of course, this is notable because Kayla has recently attempted to distance herself from Patrick and Brittany ... after the latter struck up a bond with Kelce's new bae, Tay Tay.

ICYMI ... Kayla unfollowed the famous couple online -- later explaining she did it to "protect" herself amid the tight end's new romance and his obvious continued bond with Patrick.

While Kayla seemingly shook off any connection to Mahomes/Kelce -- she's clearly still cool with Jackson, which makes sense based on what else we know.

Sources tell us there's no bad blood between any of the parties, and just because Brittany and Kayla's friendship fizzled after she and Trav ... there was never an expectation anyone else surrounding them would cut ties, including Jackson.

Trav, of course, has moved on ... and Kayla seems more than ready to do the same -- saying last month she's no longer interested in dating athletes, wanting a man with power instead.

On its face, her run-in with Jackson doesn't appear to have been romantic. But it is interesting Kayla is in Vegas at all right now ... suppose we'll see what happens.