Autofocus in semantic search select2 dropdowns has stopped working

When a select2 dropdown is opened, its search input normally automatically takes focus so the user can start typing. In the semantic search form, this feature was originally embraced so that users could type their way through the form. However, somewhere along the way, autofocus has stopped working, forcing the user to click or tap the search input first.

From a preliminary investigation, it seems that select2 is still *trying* to focus the search input, but something else is stealing focus immediately after that. I have not been able yet to determine the exact cause, let alone to remedy it.