And after several back and forths between Movies Anywhere and Warner Bros, Warner finally issued a new code to me this morning that redeemed properly this time.
I checked my various MA-partner retailer accounts and found that not all of them provided the new 4K version:
Apple TV/iTunes: HD
Vudu: 4K
FandangoNow: 4K
Prime Video: HD (no 4K currently available)
YouTube: Unable to locate
Apparently, some MA codes for Willy Wonka are redeeming on MA with the wrong version, only offering the HD version on the Movies Anywhere app/service (MA will show 4K/5.1 and "This device does not support 4K playback."), yet the correct version is still going through to retailers like Vudu...