IB tracks at the position level. So what you’re seeing in the IB chart is your combined position which is not any sort of exchange price. If you enter a buy limit for a short spread that’s saying that you will buy the position back at that price which generates a debit (a positive number). If you enter a negative value that tells IB you want to open a credit spread (a negative number) which would get you shorter your straddle. The chart is misleading because it just prints the position price. A stop loss would be a debit that’s greater than the credit you received. For example you enter a stop loss at 5. That means that volatility has increased and its now more expensive. The net trade would be -3.87+5=1.13 debit or loss on the trade.

In my experience this gets a bit counterintuitive especially when your spread goes OTM. One way to get a feel for this is to trade manually in your IB demo account.