Nios2 gen2 interrupts getting disabled

I have just ported a project that's been working for years with a Nios2-gen1 processor to the new Nios2-gen2. Everything seems to be working fine except for one strange problem, I'm finding my interrupts are getting spontaneously disabled for some reason.

In debugging this, I've found that occasionally when an interrupt occurs (from the internal interrupt controller) the estatus register is zero. estatus should have a copy of the status register at the time of the interrupt, and since the interrupt can't occur if bit 0 of the status register is clear, then I would never expect estatus to be zero on entry to an interrupt.

I've checked the exception register and it indicates that the cause of the interrupt is the IIC, so it's definitely a hardware interrupt causing the exception.

Does anyone know if this is a known errata on the Nios2-gen2 processor?

For the moment I'm just forcing estatus to 1 in my interrupt handler, but I'd love to know what the root cause of this issue is.

Thanks,

Steve