Bug? Object.proxy_collection is been set to linked target object even when target is not a collection

In 2.79 linking in a object (‘Cube’) then making a proxy of it will create a new object
Cube_proxy with Cube_proxy.proxy pointing to Cube and Cube_proxy.proxy_group == null.

in 2.81 Cube_proxy.proxy points to Cube and Cube_proxy.proxy_collection to Cube.

Reading the docs this look like a bug