WP34S on iOS (iPhone) clears display when put down

[Aussie Susan]

(Apologies if this has already been asked but I couldn't find it in a search of this forum.)
I am running the latest (as per the App Store) version of the WP34S on my iPhone and generally I am very happy with it.
However sometimes when I complete a calculation and put the iPhone down on my desk (gently!) the display becomes "0.00".
As far as I am aware, I'm not pressing anything on the screen as I put down the phone.
Also this seems to happen randomly - at least I can't seem to force it to happen and it does not happen all of the time.
My questions are:
1) Is this a bug or a feature?
2) If a feature then how can it be turned off?
2) Is there some way to get back the number that was previously displayed?
Thanks
Susan