To reproduce:
First, set up two command blocks. One that fills an 8x8 or greater area with air, and one that fills it with shulker boxes. Make sure this area all inside one chunk.
Activate the second one, filling an 8x8 area with shulker boxes.
Then, activate the first one, filling it with air.
Then, place a chest in the area that used to have shulker boxes.
--> Observe that it is a chest, but is rendering as a shulker box.
If you do this inversely, with filling chests, and placing shulker boxes, then you will get a crash and the following error:
Description: Rendering Block Entity
java.lang.IllegalArgumentException: Cannot get property bcu{name=facing, clazz=class eg, values=[north, east, south, west]} as it does not exist in bci{block=minecraft:purple_shulker_box, properties=[facing]}
at bci$a.c(SourceFile:144)
at cdb.a(SourceFile:70)
at ccz.a(SourceFile:134)
at ccz.a(SourceFile:122)
at cba.a(SourceFile:674)
at cau.b(SourceFile:1178)
at cau.a(SourceFile:1086)
at cau.a(SourceFile:944)
at bob.b(SourceFile:822)
at bob.a(SourceFile:382)
at net.minecraft.client.main.Main.main(SourceFile:140)
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@Marcono1234
@Pokechu22
MC-82703 is the actual error.
The crash is probably caused by MC-123363.
They seem to be oddly related, as whenever you place a shulker box in the same area, it probably thinks it's still a chest, and therefore doesn't have the state of up. That's why it works with shulker box -> chest, and not chest -> shulker box. Might need some more testing though. The two are definitely related though.

Alright, thanks.
Yeah, it's probably the crash from MC-123363. I'm not sure if it's the same cause as that issue though – if it's the wrong type of block entity renderer being used, that explains the crash itself, but I'd say the condition that's causing it is different. But I'm not entirely sure how the system has changed post-flattening.
Does MC-82703 describe this issue?