When saves file with .ora, transparancy mask disappear in the next file open, completely delete any area with "black-painted" part inside the "marked" layer. *englsih isn't my primary language*
Yes, I can confirm the same issue on Ubuntu 14.04 as well with Krita 3.1.4 Alpha appimage.
OpenRaster doesn't support masks. If you want to save masks, save to .kra.