Stock Item reindex acts as "update on save" even when using "update on schedule"

Preconditions

1. Magento 2.3.0
2. Inventory indexer set to "Update on schedule"

Steps to reproduce

1. Use import-export feature to import any MSI stock items

Expected result

1. Stock items are saved
2. Reindexing is handled by scheduled cron

Actual result

1. Reindexing is handled immediately after import, then again by scheduled cron

Why this happens

The underlying impleentation for all of the import strategies (replace/append/delete) uses the following interface for handling the save operation, which has a plugin attached to it to ensure that a reindex takes place.

# vendor/magento/module-inventory-indexer/etc/di.xml
<type name="Magento\InventoryApi\Api\SourceItemsSaveInterface">
        <plugin name="reindex_after_source_items_save" type="Magento\InventoryIndexer\Plugin\InventoryApi\ReindexAfterSourceItemsSavePlugin"/>
    </type>

The plugin does not test the current indexing strategy (save/schedule) to decide if the reindex should happen immediately or be scheduled for later.

Start with vendor/magento/module-inventory-indexer/etc/di.xml and Magento\InventoryIndexer\Plugin\InventoryApi\ReindexAfterSourceItemsSavePlugin, then trace how the import strategies call SourceItemsSaveInterface. Done means imports using "Update on schedule" do not trigger an immediate reindex, while scheduled cron still handles reindexing.