Binding to custom session object key and then referencing produces Bad_Stale quality

Ignition
1

v8.1.5

I created a custom session prop which was an object for e.g.

session.custom.thing =
{
 'param1': 1,
 'param2': 2
}

When I bound session.custom.thing to a View custom prop and then bound another custom prop to read one of the keys in thing e.g. thing.param1, the quality of that binding was Bad_Stale.