I think I had the exact same issue with my C-71, It was constantly locking the action open with ammo remaining in the magazine. SBI designed the buffer pin to be slightly undersized (or the hole to be oversized), to aid in removal, but I think it was vibrating enough with every bolt impact to shift the bolt stop up slightly. Once I replaced the pin with a fiber one, the issue went away completely.
I think I had the exact same issue with my C-71, It was constantly locking the action open with ammo remaining in the magazine. SBI designed the buffer pin to be slightly undersized (or the hole to be oversized), to aid in removal, but I think it was vibrating enough with every bolt impact to shift the bolt stop up slightly. Once I replaced the pin with a fiber one, the issue went away completely.