Pool aliases redirect the reference to an alias pool to a different pool. This however doesn't work for the start pool.

how to reproduce:

I've attached a data pack broken.zip containing pools jacobsjo:pool_a (red concrete pillar) and jacobsjo:pool_b (blue concrete pillar) and a structure jacobsjo:test that uses jacobsjo:pool_a as start pool and

"pool_aliases": [
    {
        "type": "direct",
        "alias": "jacobsjo:pool_a",
        "target": "jacobsjo:pool_b"
    }
]

The jacobsjo:test structure is generated every 2x2 chunks.

1. Create a world with the broken.zip datapack.
2. Red pillars generate in the world. - The reference to jacobsjo:pool_a did not get redirected.

To verify that the pool aliases work for pools that are not the start pool, I've attached a working.zip datapack that is identical to broken.zip except that is uses a separate start pool that then references the jacobsjo:pool_a pool.

1. Create a world with the working.zip datapack.
2. Blue pillars generate in the world. - The reference to jacobsjo:pool_a get correctly redirected to jacobsjo:pool_b.