I´m not a chemist - far away from that but can´t this be explained by looking att the reaction as a disproportionation reaction there the reduction (forming of H2O) will be happen faster than the oxidation (forming of O2 or oxidation of some other substances with help o the free oxygen radicals). In my no chemical brain I see it like the following scenario. - The reduction process (forming of H2O) happens directly when the O atom leave the H2O2 molecule but in order to fulfil the oxidation process the free O atom has to find another O atom or another molecule. It will be a time lap between the two processes – therefor we see it as a lower ORP (due to H2O forming) followed of a rise in ORP (forming of O2 or other oxidized molecules)
I will not argue with you about it – its only a suggestion that can give an explanation to the observed measurements.
@DangerDave
I think that you got a to fast reaction in the plastic chamber and that to much H2O2 was pressed out in the water column in a speed that the secondary catalyst did not managed and therefore you get a situation rather alike dosing H2O2 directly into the water column. Not so many catalyst or lower concentration of H2O2 in the plastic chamber will probably be the best way to go.
Sincerely Lasse