BalancingPool needs some warmup phase after idling for a minute

Test setup: I am using a BalancingPool(nrOfInstances = 2) to run long running blocking operations from a play framework application (akka 2.6.1 in play framework 2.8.0).

My problem is that when the BalancingPool is idle for one minute, it does not balance anymore but just uses a single actor, instead of the defined two. Only when I send it another message afterwards, it resumes to dispatch the messages properly within its two child actors.

Any ideas what could be the problem or how I could debug it further?