During our beta Mithril itself has matured from 0.2 beta, to 1.x stable and later 2.x stable. They had their own set of changes, which had limited impact for Mithril components but there was a lot more impact when trying to hack around it, with rendering timing, internal vdom structure, etc. So I wouldn't be surprised that code that used to work before simply doesn't work anymore, even if we didn't explicitly change it in Flarum.
oncreate hook and work from there. Ideally you would extend the hook of the specific component you want to interact with.
Using hooks has the added benefit of being able to access the models and state specific to the component being extended.