It seems that the problem occurs only if relaxed construction is enabled (it is by default in Weld SE). If disabled, the app does not fail still the interception will not start to work until after the constructor has finished, i.e. demo.DemoClass.init() is not intercepted (this is a know limitation of Weld proxies and subclasses).

Also it's very likely not a regression as the reproducer fails even with older versions of Weld (2.3.5, 2.4.5, 3.0.0).