I wonder if MP's dramatic post-DT history was a bit of a red flag for Dave.
I would be shocked if this was the case. Absolutely stunned. Because the fact is, for as much as people in the DT fanbase like to slag on MP, no one who isn't either a fan of one of MP's projects or a Blabbermouth reader actually knows about most of his controversy, or if they do know, they don't really care or think any of it is a big deal.
Think about the specific incidents that DTF users cite as "drama" for MP. Most of it surrounds a band breakup, which was actually one of the less acrimonious band breakups in music history if you actually look at it. Look no further than Mustaine who just recently seems to have mostly gotten over being kicked out of a band in 1983. Or, if you must, look at the Queensrÿche debacle. Or the Pink Floyd dispute back in the 80s. All that happened with MP is he liked some negative Facebook comments about JLB, said some less-than flattering things about him, and had a little media spat with him that was mostly manufactured controversy by Blabbermouth and was quickly resolved.
The other incidents that constitute MP "drama" are basically all just ill-conceived social media posts. In the grand scheme of things, no one cares about ill-conceived social media posts by some drummer of a band they sorta like. Certainly Dave Mustaine, the veritable king of making ill-conceived statements in public, doesn't care about MP's Facebook page. This is the guy who had no problem talking about how he thinks that every mass-shooting is a government conspiracy. I don't think he cares that MP got mad one day and said some mean things about the speed of service he got in a hospital.
I get that MP's not a perfect human being. He's made some very public mistakes, and it's fair to call him out on those. It's also fair if you don't particularly like the guy knowing those things about him. But the whole "MP is so out of control, just a PR nightmare!" thing is overblown in the DT fanbase. As musicians go, his record is pretty clean. A family man by all accounts who is happily married with kids. Had a drug problem, yes, but turned it around and used his fame to, arguably, help others with similar problems through the 12SS. Close friends with a relatively well-renowned Christian rocker. No nasty rumors about drugs, sex, partying, no hint of bigotry, no abusive relationships. Just the occasional overreaction on Facebook. That's a pretty good record. Compared to a lot of musicians, it's practically spotless.
No well-known artist is going to look at that record and say "well, this guy is by all accounts an excellent musician, an excellent arranger, fairly good at helping with business and promotion, and he has a sizable fanbase, some of whom will buy anything he's involved in. But I just don't know, because all this Facebook business suggests that he could be quite a liability to our brand. Quite a loose cannon, this one." Dave Mustaine, a man who has done more cocaine than perhaps anyone alive and who is well known for repeatedly saying things that he shouldn't say to the press, often about Metallica, often about school shootings being false flag attacks by the U.S. government, is certainly not going to look at MP's record and think that.
Tl;dr: DT fans who criticize MP aren't necessarily wrong to criticize some particular incidents, but they are exaggerating his PR problems by a lot. There is no way that Dave Mustaine decided not to let MP play with Megadeth because of some stuff that he said on Facebook, because it's simply not that bad and because Dave himself is way more of a loose cannon than MP has ever been and will ever be.