Magento 2.1.9 Configurable::getUsedProducts returns a different array after product collections is cached

[apedicdev]

Preconditions

Magento 2.1.9

\Magento\ConfigurableProduct\Model\Product\Type\Configurable::getUsedProducts returns a different array after product collection is cached.

Steps to reproduce

1. Load a configurable product
2. Observe the returned array $usedProducts
3. clean the collections cache (bin/magento cache:clean collections)
4. Reload the same configurable product
5. Observer again the returned array $usedProducts

Expected result

1. The array $usedProducts should contain the same indexes->values where indexes are the product children ids.

Actual result

Before cleaning the collections cache, the returned array is a simple array indexes->values where indexes are incremental.
After cleaning the collections cache, the returned array is an associative array where the indexes are the product children ids.

[magento-engcom-team]

@apedicdev, thank you for your report.
We've created internal ticket(s) MAGETWO-83395 to track progress on the issue.

[okorshenko]

The issue has been fixed in 2.2-develop branch. Will be available with 2.2.2 release