1.3.0

Two things a real pack does that an import used to lose: the variety between floors, and the NBT a block carries.

Nothing here changes world generation.

A tall building comes in with its middle intact

A building can name several parts for the same level. A band written as range: "9,12" twice, naming a different part each time, is the pack telling the generator to pick between them on every level in that band.

The import took the first match every time, so a band of four storeys came in as the same storey four times. On a nine floor building with two alternates for its middle, that is most of the building arriving wrong.

It steps through the candidates by level now, so the plot shows what the building is made of, and it steps the same way every time, so the same pack always imports the same.

Command blocks keep their commands

A palette entry can carry tag, a raw NBT compound, and that is the mechanism behind the command-block technique: Lost Cities places the block already holding its command, and with auto set it runs where it lands and turns itself into whatever it was there to place. A pack built that way is mostly command blocks.

The import pasted the block and dropped the tag, so what arrived was an empty command block that did nothing. It carries the tag now, and so does an export, which means chests keep their loot tables and spawners keep their mobs as well.

By default the import leaves a pasted command block unable to fire. A workshop is not a world, and forty spawn commands going off while you are trying to look at a building is not useful. The command is still there and still exports.

/lcdev import mypack:main run

run pastes them live, so they fire and resolve into whatever they place. You can give keep and run together, in either order.