Due to the flattening, several generated structures will be missing several blocks when created in unloaded chunks and newly created worlds. The structures affected by this (there may be more) are:
1. Woodland mansions (Wood planks and carpets are missing)
2. Igloos (Snow and carpet is missing)
3. Ocean monuments (Wet Sponges are missing)
4. End Cities (End Bricks, Magenta Glass Panes, Dragon Head, Banners)

Note: There may be more, these are the only ones I know are affected.
Woodland mansion picture below (This is not a chunk error)