strict mode evaluation taking interface instead of class

I am getting a compilation error with my drl, that worked in 5.0.1:

Unable to Analyse Expression las.id == $las.id:
[Error: unable to resolve method using strict-mode: piag.entityinterface.ILink.id()]
[Near :

]
^
[Line: 42, Column: 30] : [Rule name='rule1']

here's the rule:
rule "rule1"
when
$las : Link(name == "GA1")
$pi : PayItem(las.id == $las.id, amount > 0, comparisonType == "ECHT")
then ...

interface ILink is empty:
public interface ILink { }

class Link extends ILink adds a few attributes including id, name, amount, comparisonType...

interface IPayItem has this getter:
public ILink getLas();

class PayItem overrides this getter:
@Override
public Link getLas() {
return this.las;
}

It seems like drools is incorrectly treating $pi as IPayItem. IPayItem.getLas() returns ILink which does not have method id(), as the message says. But PayItem.getLas() returns Link which does have attribute id.
I didn't specify anywhere that $pi is IPayItem so why does it do this?