Yeah, same for me, sucks. But, sometimes iTunes 4K upgrades take time to ripple through the system and at times they've even gone back and forth between HD and 4K a few times before settling in, so fingers crossed.
EDIT: Problem seems to only apply to iOS, there all my Disney-owned movies still appear as HD (other movies show 4K). But then I checked on my 4K Apple TV and it appears all (or at least most) Disney movies are now 4K (even really old ones like Beauty & The Beast), not only that but most seem to support Dolby Atmos and Dolby Vision as well. Awesome day!! Good job Disney for finally resolving this, it restores my faith. Now I won't have to hesitate buying new Disney movies on iTunes.