I sent a lengthy e-mail to Apple's accessibility group describing the problems I have encountered using iOS 11 with several Braille displays. I received a response from Apple indicating that they were looking into the issues. Yesterday I installed the latest iOS update hoping that these issues would go away, but no such luck. The more people who report issues to Apple, the more likely they will be to fix them.
Text Input With a Braille Display is Unreliable When Typing Quickly
When using a braille display or braille keyboard for text input, if you type quickly, the translator will not recognize all text typed. We have observed this behavior occurring both with US and UEB braille, and with both uncontracted and contracted 6-dot input. The longer the string of text entered, the more characters the translator will miss.