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…Hydrogen peroxide is a weak acid, that’s why the solution becomes acidic…
Reason has it that this would mean it becomes alkaline since the acid (H2O2) decomposes .
I don’t have a definite answer but a guess: It might be stabilisers which causes the effect. H2O2 is often stabilised with a small amount phosphoric acid. Once O2 has gassed out, volume goes down and, because it’s lacking a buffer, even that small a change leads to a noticeable drop in pH.