Icondrop replacing icons

When I first started using IconDrop, I believe that in both seating plan mode and gradebook modes, it allowed me to drop more than one icon (so if I used it 3 times, I had 3 icons in the box), which was a fantastic feature. But I noticed recently that when I drop an icon, it replaces any icon in the box, rather than adding to it. For instance, I used to use it a lot to add behavior related icons to an attendance column, but now it seems to replace the attendance. It is no longer as useful as it was when it was first introduced and I've had to mostly give up on it, because I tend to use more than one icon at a time, which is making use of my favorite app more cumbersome!

Did I change a setting somewhere to cause this behavior?

Yes, you changed its behavior.
Tap on the three dots in iconDrop and enable > Add icons instead of Replace