Were the 8 items still on the SO? Sounds like you had to put them back on, which would suggest that at some point the quantity was zeroed out, whether by human error or a bug. Also, how do you release? Do you use SO-E or do you do it inside the SO? Were the 8 other items of the same item number on the same line or were they separate line items?

Unfortunately, this result is possible. IF you are using SO-E and you go to release a line with multiple units, you can enter 1 in the release quantity and 0 in the backorder quantity and it will write back to the SO that 0 are left to ship. Then, when the SO is invoiced the popup will offer to close the SO because no lines have <>0 remaining. In the absence of any other errors, I'd suspect this to have been the case. A case of the hand being quicker than the eye.