PCB troubleshooting cause

Hi,

We had a electric failure on a printed wired board after a few hours of functionning. We determined that the failure was caused by a open circuit in the PCB internal layer near to a via. We performed a microsection but unfortunately, it seemes that the chosen axis was not the best to reveal the defect. Maybe the open circuit has be destroyed by the microsection. We only see delamination near the suspected conductor. Another via shows no delamination. Please See the attached photo. Is anybody knows what happened ? is it possible that delamination has been caused by a local over voltage due to a track defect? what about the other pcb manufactured in the same batch ?

thanks.

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