$autocomplete is :
var $autocomplete = $(context).find('input.form-autocomplete').once('autocomplete');,
so it contains all autocomplete elements in the context for the current Drupal.behaviors.attach() call.
Then the code builds one single autocomplete.options for all of them, and attaches it to all the autocompletes in $autocomplete. Thus, unless I'm missing something, the data-autocomplete-first-character-blacklist set in one autocomplete spills over to the all of the others ?
Make a test that shows the bug
steps to reproduce the bug