MemoryDatasetClient._push_item() appends the caller's dict, and MemoryKeyValueStoreClient.set_value() stores the value object itself. Mutating the object after pushing it changes the stored data, and pushing the same object twice stores two references to one object. The file system and SQL clients store independent snapshots, and they reject non-JSON values that the memory client accepts.

Store a JSON snapshot of each dataset item, and a copy of each KVS value (or serialized bytes, once #2297 lands), so the memory client behaves like the other backends.

This will be a breaking change.

JS implementation: apify/crawlee#3812, apify/crawlee#3831

✍️ Drafted by Claude Code