Oh, I did not know that - I thought Loops writes insulin to Health in open loop. The 30-400 idea makes sense, but as @Trying noted, Loop would still zero temp after a bolus. I think it would not be too difficult to modify the code so that the programmed basal is enforced in closed-loop no matter what (this may be one or two lines of code) - let me know if you are interested, would be happy to take a look.
You do not need to actually deliver the bolus to see how the IOB decay curve looks like. Just tap on the Insulin Model, and Loop will show you the IOB decay curve for the four choices. If you made any customization, that would be automatically reflected in the curve shown.