I'm facing the same problem - I started to export and email the export file to my account every time I add or change a password. I will implement a merging functionality on my PC to sum up all the export files in the correct timeline and generate a complete importable file of my passwords that I will than hold in my cloud so I can restore my vault any time I need.
I still don't understand how the App can actually lose entries in the list. I'm using an iPhone 5s with the most recent iOS.
Several attempts to diagnose and reproduce this issue have failed, so far, to yield the described effect/issue (the app has always worked perfectly on my test devices and those of the people around me). As such, this issue has been difficult to pin down or eradicate.
Consequently, the current effort is in a rewrite of the app. The rewrite of the iOS app is underway and progressing, though there is no concrete date of when it will be ready for prime-time.
Given that this issue is relatively rare, the best advice I can give so far is to keep your sites backed-up, especially if you use non-generated tokens. If you use purely generated tokens (ideal use case of Master Password), then you shouldn't have to worry too much, and a loss of sites is merely a convenience issue, since sites can be easily recreated with identical passwords.
Even if someone - like me btw. - uses only generated passwords, we loose login names and the corresponding iteration of the password. And they differ a lot - I have iterations from 1 to 26 (and counting), depending on security level and expiration time for the passwords.
So: yes, my passwords can be generated again, but I have to rightly guess the iteration they are in and to remember the correct usernames ...
So even with generated passwords this is not only a convenience issue, at least for me.