For future users who come across this thread, just want to clarify that web containers in a
preload.json is NOT supposed to work and it was never designed to work. And this is not a bug.
The biggest benefit of Jasonette is that it's completely declarative and composable. And being composable means a lot of the behaviors are generative and you won't know how it would work until you actually try.
So the simple solution for this is don't use web containers in preload.json. The whole point of preload screen is to have a simple screen that's there only until the next page loads. There are plenty of creative ways to display loading screens that go well with the app theme.
This may change in the future if there's some critical reason or huge benefit for supporting web containers in preload.json but for now let's use it for just that--a simple preload screen