If you issue a bolus from Loop and then decide you want to cancel said bolus by suspending the pump it will for a time (I didn't count, but it was a few minutes) show you that undelivered bolus as pending insulin.
Here's an example I caught with screenshots.
After cancelled bolus and after editing carb entry for meal

After a minute or two the pending dropped off and suggested a new bolus

It made sense to me why it was not suggesting a new bolus, and with the patience of those few minutes I got a new recommendation, but I could see how someone wouldn't realize this is how it works and perhaps be confused.