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When I activate ajax for my views, and enable term autocomplete exposed filter, it looks like that form submit happens too early, when I select some option from autocomplete and hit enter. So, if I write "Пу" and the autocomplete suggests "Пушкин" and I try to select it using my keyboard arrows and enter, the form is submitted with "Пу" and throws validation error. You can reproduce it here:
(try to type "Пу" in "Автор" field)