Random preload failures even when the model is preloaded

I get this every once in a while. I'll load a page, and it'll say that the model must be preloaded with X even though that query calls `preload_x`. Without changing code, sometimes just refreshing the page fixes it. I've ignored it because it's a really weird intermittent issue, but now @robacarp has also seen this. So there's some weird race condition somewhere happening.

Start by reproducing the intermittent failure described when a page calls `preload_x`, comparing it with a page refresh that makes the error disappear. Trace the model preload path and look for the reported race condition; done means the preload no longer incorrectly reports that the model must be preloaded.

Written by the indexing model from the issue text.