Thanks for reporting.
This is our issue 326
BUT, it is reported also for stock/unaltered-PRS-x50 readers and may or may not be related to PRS+.
Is there any pattern on this on "many occasions"?
I, for myself, use my PRS-650 with current beta-pack.
I have NO memory-card in my reader,
have set standby-picture to titel-page of the current book,
and AutoStandbyTime to 5 minutes.
I don't use any management software like Sony-Reader or calibre, but do simple side-load my ebooks. I don't even care of save eject - mostly while programming.
I don't use highlighting and annotations, but occasionally (if reading in English) the dictionary.
I don't use any PRS+ Apps at a regular basis, because they are known for instability.
Most of the time I put the reader into Standby with the power-slider when finished reading. And I faced battery drain only once or twice in the last two years.
So really hard to track this down. Is there something very different in your reading/using habits?