Oops, meant to leave untriaged.

This appears to be hitting the code that makes sure not to count page reloads as meaningful omnibox interactions.  In other words, because https://hstspreload.org reloads the same page that is currently displayed, step (3) effectively isn't retraining the omnibox to do anything.

For example, the following steps work:
(1) Open a fresh profile.
(2) Type "hstspreload.appspot.com" into the omnibox and press enter.
(3) Open a new tab.
(4) Type "hs" into the omnibox and select the "https://hstspreload.org" completion.

Now focus in the omnibox (in this tab or any other) and type "hs".  hstspreload.org the inline autocompletion.  In other words, the omnibox learned correctly after a single lesson.

I think it'll take a lot of persuasion to convince me that things that are equivalent to reloads should cause a page to be suggested more strongly.  Often people reload pages simply because something is broken on the page; it doesn't mean they want it to be offered as a suggestion more often.

I'm going to close this bug as working as intended.  However, I understand your root point--that it should be easier to teach the omnibox your preferences.  I'd love if you can come up with a more realistic example that'll better indicate something that we should change.

Alright, interesting to know.

I do know that hstspreload.appspot.com came back several times as the autocompletion for "hs" on my synced work profile, even though I always selected "hstspreload.org" (and often deleted hstspreload.appspot.com).

I'll file a bug if I ever get lucky enough to isolate anything.