Assets are deleted and memory reclaimed periodically or if memory pressure is high automatically. To force this, one can use Resources.UnloadUnusedAssets to unload and reclaim memory that are no longer referenced by any active object. This is by design.
More Info here:
https://docs.unity3d.com/Packages/com.unity.addressables@1.3/manual/MemoryManagement.html#when-is-memory-cleared