For the record, I reached out to IBKR support (before posting this thread). At first, they too were stating that the order filled at -$0.10, but I insisted they dig deeper and they realized that was not the case. They went through the audit trail, and confirmed that it filled at -$0.20.
They could not explain why this happened. They said they would escalate the ticket, but I have not heard back from them.
Now that I am looking at these trades more closely, I am finding that the orders are often not closing for the limit price.
I am open to the idea that I am placing the order incorrectly, but if so, I have no idea of what the correct way is.
I think it's a display problem on the IB platform. If all of the legs on the combo did close at -0.20 price and your P&L corresponds to it, it should show $-0.20 not $-0.10. That $-0.10 is just the wrong price displayed. You have to realize IB is constantly doing "updates" to their platform and f***up happens all the time. The developers that they hire are not the best. It took them so long to just fix some simple bugs in the "Strategy Builder".