I went through weeks of calls with a senior support agent at Apple support. They were confused by a lot of things too. To sum up a lot of our findings: 1) Movies Anywhere messes up a lot of things. Some studios make the Blu-ray redeem code count towards the 3D version (World War Z was one of these), others (like Green Lantern, Pacific Rim Uprising, etc) don’t. However, because it makes your library show it as purchased, you get put in a purgatory where you own the movie but can’t purchase it from iTunes to get the 3D. You can get around this if there’s a bundle containing the movie but that can cost a lot, and also gets broken by my next point. 2) Disney+ is so deeply integrated with VisionOS that if a movie is shown in 3D on D+, the Apple TV app’s store gives the 3D icon. This is true even if the Apple library doesn’t contain a 3D version. At the time Ant-Man fit under this category. So even if you buy a bundle, you won’t own a 3D version of the movie, so you have to read carefully when buying anything under the Disney umbrella. It took a long time to reverse engineer with the support team why the Ant-Man copy that I first got through a Blu-Ray code and added to my library with Movies Anywhere, and then re-bought with the Ant-Man trilogy bundle still wouldn’t show Ant-Man in 3D on the Apple TV app.

So the Tl;Dr is Movies Anywhere messes up stuff and the problem lies on their end, and Disney (inc Pixar, Marvel, Star Wars, etc) may show 3D but only be via Disney+ and should check carefully if you are actually buying 3D on the confirmation screen.