I am following this thread and wonder if you have any more thoughts.
Im actually thinking maybe the Iron Oxide may have been oxidised, hence resulting in Oxygen being consumed in the system.
Another possibility is gas solubility, at higher temperatures (65 and above), Oxygen is able to easilly dissolve in the water, resulting in a decrease in pressure. However, if my research is correct, at higher temperatures, Hydrogen Peroxide and Water should more easily release Oxygen, contradicting this idea.
The possibility of a leak at higher temperatures has been ruled out as if there were to be a leak, the pressure should have equalized at some point. In my case, the pressure remained below atmospheric pressure.