I think when you freeze the milk, it's likely that different parts of it get separated out. If I'm reading this correctly, your MIL is taking only a portion of what's frozen at a time, meaning that during different feeds you'll get different quality of milk. Some might be mostly water, others night be more concentrated. Perhaps you need to change the quantity that you freeze in 1 bag/container, and ask MIL to use whole thing in one feed.
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