The ex-'Deadpool' star sat down for a chat on 'The Adam Carolla Show' -- where he revealed part of the reason why he believes he might not be in the forthcoming second sequel (aka, 'Deadpool 3') and, in his eyes, it all has to do with RR hating his guts for some reason.

He recounted a story in which he claims Ryan berated him during the filming of a scene in one of the first 2 films. He says Ryan, playing Wade/Deadpool, improvised some super mean lines while addressing him in character as Weasel ... but TJM thinks he was actually just operating under the guise of acting to take some pot shots at him personally.

Miller says Ryan, as Deadpool, told him/Weasel that no one really liked him and that he wasn't funny ... and that he only served as temporary comedic relief that nobody (including the audience) cared about ... basically calling him unnecessary for 'Deadpool' at large.

It was a pretty awkward exchange, so says T.J. -- who claims the crew was just as stunned by the on-set teardown which felt very real. Since then, he says, his relationship with Ryan has soured -- and, obviously, it has with the studio since he isn't going to be in 'DP3.'

But the way T.J. frames it here ... he makes it seem like he's the one who wants nothing to do with Ryan -- saying he wouldn't work with him ever again, going on to openly wonder why he dislikes him so much ... and going further by suggesting RR was jealous/insecure.

T.J. has a lot to say about what he perceives as flaws in Ryan's character ... especially as he got incredibly more famous with the Deadpool gig. He even makes some observations about Blake Lively and their image -- or "brand," as he and his pals call it -- as a couple.

It should be noted ... right as 'Deadpool 2' was coming out, Miller was dealing with very serious sexual misconduct allegations, which he denied. Still, you gotta imagine that may have played a part in either Ry and/or studio heads weighing his future involvement.