Why does Impress change the background when I save a presentation?

Sometimes after I complete a presentation using several different background images on different slides, Impress changes the background of all the slides to one of the images I used. This doesn’t always happen, but it happens often enough to be a real annoyance because I am forced to go back through the entire presentation and change the background of each slide to the particular image I want. Of course, even when I do, there is no guarantee that they will not be changed again when I reopen the file later.
Libreoffice / English / Impress
Ubuntu 15.04 / English


I’m not in the same environment. I have never encountered this problem. A search of identified bugs gives results only for export to other formats that ODP. Would you be in this case?