I have this problem quite a lot, too. I'm trying to use iPads and dictation on a professional level for my architectural and engineering firm. This is rather difficult when dictation randomly kicks me out and/or deletes everything that I have just dictated.
I have discovered a poor workaround. It works but it is very inconvenient. At least it allows me to function under the circumstances.
It works as follows:
If you're working in one of these deletion zones, like this form here or eBay or any other place where you know that your text is going to get deleted as soon as you hit "done", don't ever hit "done". Instead, finish your dictation by tapping elsewhere on the screen. Put your cursor someplace else. That will finish the dictation but prevent the deletion. Of course the hard part, is when dictation kicks you out randomly, then it will delete what you just dictated. The trick is to move your cursor before it kicks you out.
Consider moving the cursor like that sort of like an auto save. In my experience, don't go more than about a paragraph, or you risk having dictation kick you out.
As I said, this is a very inconvenient workaround, but at least it allows you to function.
If Apple can solve these two problems, it would vastly improve the Apple experience and productivity.
Without solving these problems, it pretty much relegates dictation to a useless novelty. Not good for anything serious.
This problem is consistent with all of the iPad Airs that my company has, also on the couple of iPhone 4 and 4s that I have access to.
