Lids developed stubborn fungal growth

Initially I quite liked this product. I used them as lids everyday over the many cups and jars I had at home. Always hand-washed it with care with good quality soap. But later-on they started developing black, fungal mold-like things. I tried using scrubs, baking soda and all the DIY clean-ups I could find, but there was no stopping this stubborn, black, growth. It almost seems like the growth is from the inside material itself, even if I scratch with my nails this growth does not go away. Plus the cups started stinking too. I was no longer sure if that was because of the growth or the silicone material itself. So I'm planning to discontinue using them for my child. Please see the pictures to get an idea.

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