As stated in summary when a modded block entity is in a chunk all block entities in the same chunk are lost when a pre-1.18 (21w43a) world is updated to 21w43a or newer.
Can only assume this is because modded block entity types don't provide a datafixer type because they don't exist in any of the schemas.
I know this isn't really a minecraft issue but thought I'd open it anyways just in case it's something mojang would like to fix.
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Sorry for the delay, been a bit busy. Here is a world backup which should help reproduce the issue. When opened in 1.17.1 vanilla you should see a double chest placed on lime concrete with a single stack of lime concrete. On the purple concrete is a wood chest from my mod Expanded Storage which has a stack of purple concrete. ( but that doesn't really pertain to the issue much )
If you open this world in 1.18 (pre-5 as of writing this) instead you shouldn't be able to see the vanilla chest despite it being there, also the items in the inventory are gone.
The world:
[media]
I believe the root cause of this is the use of tagged choice / tagged union types for block entity IDs. This is acceptable for vanilla because it has a fixed, hard-coded list of IDs, but it is woefully inadequate for mods (regardless of modding platform) because it requires knowing all block entity ids before datafixers are compiled.
Could you please provide a world that can be used to reproduce this?