Fail to set multiple adapters active

[dyxohjl666]

                self.base.load_adapter(domain_adapter, set_active=True) #load "mlm"
                self.base.add_adapter("len", config="pfeiffer", set_active=True) 
                self.base.set_active_adapters(ac.Stack("mlm", "len"))

I just discover that I fail to set "mlm" active with above codes.

This is I got with adapter_summary(). However, when I load only "mlm" without other adapters, it can be set active. I'm confused about this. How can I set both of them active？