Trigger on update doesn't fire

I have two objects: object A and object B. There is a relationship between these two objects. There is a "On Update" trigger on a relationship field on Object A which does some calculations. When deleting (detaching the relationship on record level) the relationship on object B, the trigger on object A does not fire. Is this as designed? Removing a relationship should fire the trigger, because the record has been updated...

The tests above are done on a private cloud installation running 2.1.0.2.

When testing it on a private cloud 2.2.0.1 installation, a "java.lang.NullPointerException" error occurs :

[2014-05-08 23:57:06,237] ===> Error in thread ajp-apr-8009-exec-8 at 05/08/2014 11:57 PM

[2014-05-08 23:57:06,237] java.lang.NullPointerException

        at com.rb.core.ui.web.cell.GridControl$GridDataSource.getTempData(GridControl.java:812)

        at com.rb.core.ui.web.cell.GridControl._x10(GridControl.java:541)

        at com.rb.core.ui.web.PageSection._x10(PageSection.java:455)

        at com.rb.core.ui.web.WebPage.appendTabbedRuntimeHTML(WebPage.java:1092)

        at com.rb.core.ui.web.WebPage.getRuntimeHTML(WebPage.java:847)

        at com.rb.core.logics.servlet.MainServlet.doGet(MainServlet.java:121)

        at javax.servlet.http.HttpServlet.service(HttpServlet.java:620)

        at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)

        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)

        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)

        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)

        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)

        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)

        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)

        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)

        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)

        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)

        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)

        at org.apache.coyote.ajp.AjpAprProcessor.process(AjpAprProcessor.java:190)

        at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)

        at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2441)

        at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2430)

        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)

        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)

        at java.lang.Thread.run(Unknown Source)

any idea?