RESTful clients are not being shutdown (Big Problem)

RESTful clients are not being shutdown.
At the moment we have 1000+ idle RESTful clients, which are idle since yesterday. (Started since we last restarted the servoy admin)
It is a big problem, because it keeps growing and growing until it hits the max memory on the Java process and then everything is stuck.

Our configuration:
RESTful Web Services Plugin

rest_ws_plugin_client_pool_exhausted_action: grow
rest_ws_plugin_authorized_groups: REST
rest_ws_plugin_client_pool_size: 25

Web.xml:

30

Why are the unused clients not automatically removed?
What can I check?
Are there more settings to consider?

Rest clients are supposed to be destroyed when the calls returns? What can cause that they are not destroyed when idle?

Server Information:

Servoy version 6.0.9 -build 1239
Port used by RMI Registry: 1099
Repository version 40 (127E8A55-C3F0-4B1F-8418-30D52BBDEB05)
Current time: Mon Sep 18 12:23:36 CEST 2017 
Uptime: 14 hours 42 minutes 25 seconds 

User Information
Logged in as: axsupport 

JVM Information
java.vm.name=Java HotSpot(TM) 64-Bit Server VM
java.version=1.7.0_17
java.vm.info=mixed mode
java.vm.vendor=Oracle Corporation 

Operating System Information
os.name=Windows Server 2008 R2
os.version=6.1
os.arch=amd64