I'm experiencing the same problem as described here: http://www.zkoss.org/forum/listComment/19009
My example is:

<tab apply="org.zkoss.bind.BindComposer"
viewModel="@id('vm') @init('my.TabModel')"
label="@load(vm.title)"
sclass="@load(vm.status.dirty ? 'tab-dirty' : 'tab-clean')"/>

The 'tab-dirty' and 'tab-clean' classes are applied properly when vm.status.dirty flag is toggled, but the ZK's standard class z-tab-seld is lost whenever the sClass is changed via @load.

My workaround is to have
sclass="@load(vm.status.dirty ? self.selected ? 'z-tab-seld tab-dirty' : 'tab-dirty' : self.selected ? 'z-tab-seld tab-clean' : 'tab-clean')"