Individual +/- is not a reliable metric. It's just how many points per possession when the guy is on the floor and off the floor. The other 9 players on the floor with him and off the floor without him have a lot to do with that number. The most one can say for a player with high Def +/- is that he's not a total liability defensively that drags down the whole team's defense. Players like Jokic , Sengun, and Sheppard are perceived to be defensive liability that some people say "would be played off the court in the playoffs." Jokic definitely have not been played off the court. I don't think Reed will either.
It's still a bit early in the season, so anything can happen, but in the initial 14 games of the season, there's a pretty solid comparison.
Last night's second half is an interesting case point for Sheppard's ongoing development. He had that barrage in the first half, but second half it felt like he was more just a decoy and floor spacer. He also looked like he was gassed from the minutes, as evident by IIRC an airball 3 followed by missing 2 FTs late in the game. I don't mind it because I've always wanted Ime to play Reed more even if it leads to some regular season losses. But it will be interesting going forward how he meshes with the starters, and how he will handle bigger minutes night to night.
I've seen guys do that in my life, but typically it boils down to either your mechanics or routine/ritual (which helps concentration). Reed's mechanics are exceptional. I think he probably rushes himself and doesn't take that moment to gather himself to focus on the task at hand. A routine or ritual allows you to slow down your breathing, collect yourself and then shoot your shot. The other factor is he hasn't shot a lot of FTs in the NBA and he should get better, but hopefully he develops a routine that helps him relax before he shoots. When you watch KD he has a very specific and repeatable ritual, and his results speak for themselves.
Adding all levels of pro ball (including preseason and summer league) he's shot 77% from the line with extremely infrequent trips, less than one a game. He shot 83% at Kentucky, on basically 2 FT's per game. So roughly 80% between all levels, certainly not bad but also nowhere near as good as he should be. He did shoot 89% his junior and senior years of high school, so I would hope he can get back to that level if he starts getting to the line more regularly like he did then.
I will say raw +/- can be used in the extremes with a decent sample size. For the Rockets, Sengun, Durant, Amen, and Reed (not counting 1st 2 games) all look like good players such that it seems like Rockets almost always have 2 good players on the court causing few extremes.
That was certainly true of his FTs at the end of the Magic game. He went so fast that I assumed he was doing it to avoid getting nervous. But maybe as he matures he'll do it more like KD, with a motion he can get into before he releases the ball.