Sorry it might take me some time to get back to this bug. I was running Jessie. The key problem here is reproducing the bug. Can you install and run strace on the dovecot daemon or what ever program you think is messing up the inbox?
Use -f to follow any fields and -o to save the output to a file. If it happens so easily for you the should be very easy and would clarify a lot the exact sequence of events required.
