At pH higher then 4 MnO2 will be formed in any case, this cannot be prevented. In lower pH Manganese-II is built up and MnO2 get dissolved. Right at very high pH Manganate MnO42- is also formed, but still some manganese dioxide.
Some MnO2 is created from the manganate with an reaction on the glass wall of the flask or beaker at high pH. Probably some catalytic effect.
2 MnO42- + 2 H2O => 2 MnO2 + 4 OH- + O2