The Multiple Destination for Task event is concatenated when used in IF statement, if the same SmartObject is used as the Destination outside of an IF statement it works as expected.
Destinations for the task is concatenated into a single destination, instead of multiple destinations.
This issue will be fixed in K2 Cloud Update 12 and K2 Five 5.4 expected in 2020.
As a workaround, do not use the IF inline function and instead have multiple destinations and use the Recipient rule to validate which destination gets the tasks.
Using the workaround it is hard to distinguish between the destinations when using the same SmartObject methods with different inputs, for example. Testing will be needed, and an Investigation item was logged to implement destination naming similar to how groups work where you can specify a name for the destination slot.