Hi Ross,
The key is rocked a little bit with the low side on the spacer side. I will have to file a bit when reassembling.
Be sure to be careful on the next one. What I think is likely happening is that you are not tightening the stack (balancer, spacer, slinger, lower gear) together because of that rocked key. What you were doing was tightening the gear against the key like a wedge, which could keep it from seating and even allow it to move a little. Since it was loaded unevenly, when you tightened the bolt with the spacer against it, that's likely what broke the spacer.
I doubt there is an issue, but I would also look close at the lower gear and make sure you didn't muck it up a little at the keyway
When you fit it next time, I'd replace the key for peace of mind, then make sure that the gear goes on without rocking the key (shouldn't take more that a brass drift and small hammer), then the spacer fits on the key (check before putting the slinger and timing cover on). If you do that, it will load all the parts evenly and you will have no issues.
