res.reply doesn't use user's display name if they joined flow after Hubot started

[wenslayer]

Hubot Script

robot.respond /magicword/, (res) ->
  res.reply 'open sesame!'

Repro Case

• Start Hubot
• Hubot joins a flow that Joe is already a member of.
• Joe issues a keyword that triggers a response; e.g.:

  Joe: hubot magicword
  Hubot: @Joe: open sesame!

• Mary gets wind about all this cool stuff happening in this flow, and so joins it.
• Mary tries the same thing:

  Mary: hubot magicword

Expected result

Hubot: @Mary: open sesame!

Actual result

Hubot: @123456: open sesame!

• The 123456 corresponds to the user's Flowdock user ID.
• The @123456 doesn't resolve to an existing user and so Mary is not properly @-mentioned.

Workaround

• Restart Hubot