[DeepThought]

A few possible things I can think of.

Creating an equalization debt on the way down till your e-tubes shut and you can't equalize any further. Equalizing twice/second sounds like a good frequency to prevent this, but if the eqalization volume isn't enough you could still create a debt.

Once I dove with a strong irritation to cough. While resisting coughing underwater I often couldn't equalize eventhough I didn't miss an equalization and had tons of air as it was shallow recreational diving. I became aware that some muscles tensed up in the back of my throat/soft palate and when I consciously relaxed them (or coughed underwater) the euqalization barrier was removed.
Maybe when you are far below RV with your mask pressing there is some factor that tenses those muscles.

You can try fractionating this/isolating or playing with variables.
A few examples:
Try a dive with a Sphera or nose clip and no mask.
Try replicating some of the conditions in a shallower FRC/empty lungs dive - going shallower doing the same equalizations but having less air will still result in the same equalization debt per depth so in theory you are not supposed to reach the same eqalization barrier. If you reach a shallower barrier then your barrier is affected by sub RV related issues and not an equalization debt. Unless you create one only during the sub residual volume phase of the dive.
I understand that reproducing these dives or the equivalent shallow ones are not a simple matter, and there are quite a few variables. Maybe someone will post a better suspect.
Good luck in the competition and please update re. your problem.