This is a known bug. Ironically, the problem is probably caused by bringing the imps too close together. The workaround is to not overdo it when bringing the imps near each other. Please see the screenshot below.
While they might seem far apart, the heartbroken imps in this screenshot actually came together without any more bon bons being thrown. The player just brought them this close together then left the game alone for a bit until they found each other. If she had to, she would have used another bon bon to move one of the imps just a bit closer, but she wouldn't have tried to make them completely close the gap because that would trigger the bug.
Please try that. Hopefully you'll be able to work around this bug!