Based on our study of your cable construction, we suspect that the problem may lie with using polypropylene as a filler.
Polypropylene at high temperature emits carbon gas which is conductive. However, it is often used as a filler for FR cables due to cost and when it melts, the gas created is diffused into the open.
However, for this specific construction which includes a copper shield - we suspect this shield traps the carbon gas creating a conductive path and also contamination of the mica leading to failure.
As you mentioned that it passes for bigger conductors of 4sqmm and 6sqmm, the reason could be that bigger conductors have higher copper mass so they are more resistant.
We recommend using polyethylene filler material.
As we are not a cable manufacturer, please take our comments and recommendations as reference for your troubleshooting and solution.
We hope to receive your positive results.