I am having the same problem exactly as described. Only difference is I am using an iPhone 5s.
Currently discovering further detail though… I was initially having the problem while dictating text into the WhatsApp application and experiencing the problem as described. On attempting to dictate this update into your website here via my iPhone 5s I am finding that it is not cutting out after a few words but instead, on completing my text, it wipes all of my text out!
I am now in Evernote dictating this message and having no problem at all.
Just tested using voice recognition in an iMessage text and no problem there either. Next, just returned to WhatsApp and still having the problem with the speech recognition cutting off after three or four words at most there. Prior to the update from 8.1.2 (I think) to 8.2, I had used the speech recognition extensively in WhatsApp without a problem. Returning to Evernote, the speech recognition is continuing to work fine.
Will log this with Apple support. Hope this extra information helps to diagnose the source of the problem.
(Cutting and pasting from Evernote into forum webpage now!)
Cheers,
Matt