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.