Kelly threw up a carousel of pics Wednesday on his IG -- which captured a ton of moments from his birthday party earlier this week, which looked like a star-studded soiree featuring the likes of Post Malone, Tiffany Haddish, Evan Ross, Mod Sun, Amber Rose and others.

More importantly though was the fact that his ex-fiancée, Megan, was also in the mix ... and she and MGK looked pretty damn happy, despite the fact things have looked rocky for them.

Megan recently revealed that their engagement was called off -- but she left the door open for the two of them ... saying on 'CHD' that they'd forever be twin souls and connected. In that same interview, Megan confirmed she and MGK weren't ready to walk down the aisle.

Frankly, it's a little unclear where things stand between them officially -- but on the face of it, they're clearly still friendly ... and MF is cool enough with him to help him ring in the big 3-4.

The last big occasion we saw them together was at the Super Bowl ... where Meg and MGK were hanging out in Vegas post-game with Taylor Swift and Travis Kelce. No PDA between them at the time -- and ditto for the bday party -- but they weren't icy with each other either.